Creating a Horcrux is nothing to take lightly, as it directly involves one's soul. The process is split into three parts, the first being to prepare the Horcrux, then damage the soul, and the final being the casting of a spell. It's almost impossible and unheard of to undo a Horcrux once made since virtually all accounts of known Horcruxes result in their destruction. To reiterate, Horcruxes are practically permanent, so it's best to heavily consider the side effects of doing so before anyone commits to creating any.
While Secrets of the Darkest Arts is known to have the directions on how to make a Horcrux written within its pages, the material has been banned, and therefore, the concept has been left open to speculation over the years. It is said that it's too horrible of a process to be put out into the world, though some parts are known about it. For example, it's common knowledge that one must commit a terrible act that would damage their soul. The only documentation of such an act has been by means of deliberately and willingly murdering someone. However, it's unknown if a different act can be committed to obtain the same result in damage to the soul. The murder won't work for a Horcrux if it's committed as an act of self-defense, an accident, or out of mercy. There doesn't seem to be a specific manner of death that must be achieved for the murder to work, with Voldemort having used different methods, such as the Killing Curse and a Basilisk's gaze, to count toward making his Horcruxes. These murders also don't appear to need to be committed purely to create a Horcrux, with the purpose of some of the deaths having been out of revenge or tidying up some loose ends alongside the idea of using their death to make a Horcrux. The victim doesn't need to be magical, though they may need to be human since there hasn't been a case of a non-human murder creating a Horcrux. Once the act is committed, there's no way to tell that the murder was used to create a Horcrux, as many of Voldemort's victim's bodies were examined and found no trace of anything unusual or pointing towards a Horcrux.
The rest of the steps to make a Horcrux are bits of scattered information, mainly containing theories. For example, it's known that the Horcrux-to-be must be prepared ahead of time with dark magic, but the procedure to do so has yet to be confirmed, though many assume a disturbing ritual must be done. Some form of blood ritual is one of the theories, though there are far more horrendous ones thrown around that would be dark enough to match the nature of a Horcrux. After preparing the Horcrux and committing an act to damage the soul, the one remaining step would be to put the soul fragment into the Horcrux with a spell. The spell's incantation is unknown, though it's likely of Latin roots like many other spells and related to the intended action. The spell is said to sever the damaged soul fragment from the rest of the soul and place it within the chosen object, effectively completing the making of a Horcrux and allowing the creator immortality. The more Horcruxes an individual makes, the more likely permanent immortality is. However, the side effects get worse and, as losing pieces of a soul would, they become far more noticeable to those around them.
As previously discussed, Voldemort intentionally made six Horcruxes, likely because he understood that a soul could only be split so many times while somewhat maintaining one's self. The first Horcrux he made was in June 1943, using a diary he got at a Muggle bookshop, Winstanley's Bookstore & Stationers, as the object to store his soul fragment. He created this Horcrux after the murder of a fellow student, Myrtle Warren, whose death was a result of the Basilisk being unleashed from the Chamber of Secret being opened while she was crying in a bathroom stall, which was the ultimate cause of her demise as she looked into its eyes when she was about to confront the boy for being in there. It's unlikely that her murder was planned ahead of time, but instead, it was an opportunity that couldn't be passed up, especially since she was going to catch him opening the Chamber. Rubeus Hagrid was framed for the murder and expelled from Hogwarts, leaving the Chamber too risky to attempt to open again, as Voldemort's soul had this to say about it:
"I knew it wouldn't be safe to open the Chamber again while I was still at school. But I wasn't going to waste those long years I'd spent searching for it. I decided to leave behind a diary, preserving my sixteen-year-old self in its pages, so that one day, with luck, I would be able to lead another in my footsteps, and finish Salazar Slytherin's noble work."
The second was made in August 1943, using his maternal grandfather's ring, also known as Marvolo Gaunt's ring, as the Horcrux, using his father's murder to make it. He had gone to seek out his family in Little Hangleton, finding and talking to his uncle, Morfin Gaunt, then stunned him and took his wand. Afterward, he wanted to seek out his father to confront him due to his talk with his uncle, as he blamed him for his mother's death and growing up in an orphanage. He went to the Riddle house, using a common unlocking spell to get inside, where he found his father (Tom Riddle Sr.) and his grandparents (Thomas and Mary Riddle) in the drawing room. Using the wand he took from his uncle, he used the Killing Curse on them, which were reportedly found with a look of extreme fear on their faces. Voldemort returned to his uncle, framing him for the murders by modifying his memory into believing he had committed the crime himself and returning the wand, then left with the ring. Morfin was arrested by the Ministry for the murders and was sent to Azkaban for good, remarking how his father would kill him for losing the ring as he was apprehended.
The three Horcruxes following them were made in 1946 or later, with no definitive dates, after Voldemort had graduated from Hogwarts. Hufflepuff's cup and Slytherin's locket were acquired simultaneously, using the murder of an elderly witch, Hepzibah Smith, to make the cup into a Horcrux and a homeless Muggle to make the locket into one. After graduation, Voldemort took a job at Borgin and Burkes, tasked with researching and acquiring valuable items by charming people to sell them, hoping to work with dark artifacts. He had visited Hepzibah Smith multiple times during the duration of his employment, bringing her flowers and complimenting her to charm her, which would later result in her showing the young man her most prized possession: the cup and the locket. She told him these items had powers she had yet to test and that she had inherited the cup from her ancestor and paid a grand price for the locket to Caractacus Burke, who received the locket from a "ragged-looking woman" who sold it to him for 10 Galleons. The woman who sold the locket was Merope Gaunt, Voldemort's mother, which Voldemort was able to piece together himself and deemed the locket to be rightfully his after hearing its history. Voldemort returned the cup and hesitated with the locket but eventually let it slip through his fingers. Hepzibah observed a look of hunger and a flash of red in his eyes, which she dismissed as a trick of the light.
Two days after the incident, she was found murdered. Her elderly House-Elf, Hokey, admitted to accidentally poisoning the witch's cocoa by mistaking a lethal poison for sugar. However, Albus Dumbledore theorized that Voldemort killed her himself and then implanted a false memory within the House-Elf's mind, much like he did when he murdered the Riddles. At the time, no one knew the two artifacts were stolen, as the witch had an extensive collection. By the time it was discovered, Voldemort had left his job, and he wouldn't be heard from again until a decade later. The homeless Muggle that he killed to create the locket as a Horcrux seemed done purely as a means to make it, with no known meaning connected between Voldemort and the Muggle. Ravenclaw's diadem was made into a Horcrux using the murder of an Albanian peasant. Voldemort had managed to charm the diadem's location out of the Grey Lady, also known as Helena Ravenclaw, who is the Hogwarts ghost of their House and Rowena Ravenclaw's daughter. Voldemort traveled to Albania to obtain the diadem shortly after leaving Hogwarts, where he then murdered the Albanian peasant to make it into a Horcrux. Again, there was seemingly no connection between the peasant and Voldemort.
The final "true" Horcrux of Lord Voldemort was Nagini, using the murder of Bertha Jorkins to create her as such during the summer of 1994. Nagini was the only case of a living "true" Horcrux and a loyal follower of Voldemort, making her of particular interest. She used to be a human but suffered from a blood curse known as the Maledictus curse, which was obtained at birth and passed from mother to daughter, which eventually forces the cursed person to turn into a beast permanently, with Nagini's form being a snake. Bertha Jorkins, who worked for the Ministry of Magic, was on holiday in Albania when she saw Peter Pettigrew, who was thought to be dead by the Ministry. Despite knowing this, she agreed to go on a walk with Pettigrew in the woods, where he overpowered her and brought her to Voldemort. Voldemort tortured her and broke a memory charm that was placed upon her, allowing him to learn valuable information. However, this breakthrough left her mind and body permanently damaged, which left no use for her to Voldemort. He used the Killing Curse on her and used her death to make Nagini a Horcrux. She was often kept by Voldemort's side, which made Dumbledore think she may be a Horcrux, and to help in emphasizing his heritage to Salazar Slytherin, having a unique connection to Voldemort and being able to talk through Parseltongue, only made her more of an interest to Voldemort.
Despite Voldemort not intentionally making Harry Potter into a Horcrux, he became one regardless, making him the only Horcrux ever to have been made unintentionally. Harry was turned into a Horcrux with the death of his mother, Lily Potter, on October 31st, 1981. The story of his creation as a Horcrux began in early 1980 when Sybill Trelawney had a prophecy regarding Voldemort and a child that would be powerful enough to be his downfall. The prophecy goes as:
"The one with the power to vanquish the Dark Lord approaches... Born to those who have thrice defied him, born as the seventh month dies... and the Dark Lord will mark him as his equal, but he will have power the Dark Lord knows not... and either must die at the hand of the other for neither can live while the other survives... The one with the power to vanquish the Dark Lord will be born as the seventh month dies...."
Severus Snape had heard the first half of this prophecy being spoken at the Hogs Head Inn, eavesdropping through a keyhole until he was interrupted by a staff member. Snape brings this information to Voldemort, who interprets the mentioned child as Harry, prompting Snape to beg him to spare Lily in his pursuit of Harry. The Potters went into hiding when Lily was pregnant and sought out an increased security measure once they learned Voldemort would be after their child, eventually using the Fidelius Charm to hide their location by concealing the information on their location within the soul of a particular individual, known as a Secret-Keeper. They had trusted Peter Pettigrew to be the Secret-Keeper, though he turned out to be a double agent for the Death Eaters, and told Voldemort of their location within a week. Upon learning of their location, Voldemort went to the Potter's cottage in Godric's Hollow in 1981 on October 31st, intending to kill Harry. With the protections broken, Voldemort could see Lily, James, and Harry together in the same room, with James playing with Harry. Voldemort would then enter the house, confronting James as he burst into the hall. James shouted at Lily to take Harry and flee while he held off Voldemort, but James had left his wand on the sofa, leaving Voldemort able to use the Killing Curse on him. Voldemort went upstairs to follow Lily to Harry, finding them in a locked room barricaded with furniture. There, Lily had shielded the cot containing Harry with her body. Voldemort considered Snape's request to spare Lily, giving her multiple opportunities to move away from the cot, but she adamantly refused, pleading with him to kill her instead of Harry. After the third time of not moving, Voldemort used the Killing Curse on her, leaving Harry as the only remaining living Potter. He pointed his wand in Harry's face as the young child began to cry, which Voldemort found unnerving, and cast the Killing Curse. Instead of killing him, the spell rebounded back to Voldemort due to the mother's love. Lily's loving sacrifice protected Harry, effectively destroying Voldemort's body and flinging his wand nearby, leaving him in a non-corporeal shade form. Voldemort's soul was unstable from creating past Horcruxes, so the rebounded spell forced his soul to shatter once more, which splintered off the fragment that sought out the only living thing remaining in the room: the young Harry Potter. Harry was able to survive with only a lightning bolt-shaped scar, beginning the tale of the boy who lived and the pseudo-Horcrux.