Now that we know a little more about the Patronus, let's move on to the next question: how exactly do we conjure a Patronus? The first step is to focus. What do you want to focus on? Definitely not the Dementor in front of you, who is making you feel so miserable! Instead, you need to focus as hard as you can on your happiest memory, and that memory needs to be the happiest memory you have ever experienced. If you can't think of anything, you can also imagine a scene which could possibly be a very happy memory, which is what Harry Potter did when he took his Defense Against the Dark Arts O.W.L. Next, begin creating circles with your wand. The primary purpose of this wand movement is to increase your focus and concentration, which is very important to the successful completion of the Patronus Charm. Finally, give the incantation, which is Expecto Patronum. By the way, for those of you who love language like I do, “Expecto Patronum” is Latin for “I desire (or expect) my patron (or guardian).” If you've cast the Patronus Charm successfully, you'll see a Patronus emerge from the end of your wand, and you should be able to direct it in the direction you want it to go. 

Being able to produce a Patronus is a very difficult magical skill. In fact, this charm isn't even part of the regular classwork taught at Hogwarts, although one possible reason is because most students won't encounter a Dementor at the school and so will probably have no need for it. The Patronus Charm is so challenging, it's even considered more advanced than NEWT level, which are the exams that seventh year Hogwarts students take before graduation. Many witches and wizards are never able to conjure a Patronus, although that may change after going through some sort of psychic shock. Other magical folk are able to demonstrate proficiency before reaching adulthood. Harry Potter is one of the youngest wizards to ever successfully conjure a Patronus, and Amelia Bones was definitely impressed when Harry revealed during his hearing that he could even conjure a Corporeal Patronus. Another example is when Harry took his O.W.L.S., and his Charms examiner offered him an extra point (that would amount to 1 letter grade) if he could conjure a Patronus. 

No one is entirely sure why not everyone is capable of casting a Patronus, but there are several theories. Being able to focus is one of the key elements of the charm, because the charm requires the ability to focus completely on your happiest memory, and also because the Patronus itself will only last as long as you are able to maintain your focus. This skill becomes even more difficult when being faced with actual Dementors.

There is a belief among many in the magical community that only someone with a pure heart who is willing to fight for a noble cause can master this charm, and often witches and wizards who can conjure a Corporeal Patronus are elected to the Wizengamot for this very reason. We do know Dumbledore taught the charm to the Order of the Phoenix members as a method of communication, and many students in Dumbledore's Army were able to produce a Patronus, at least in a classroom setting, although it proved more challenging during the Battle of Hogwarts when many were faced with actual Dementors. It has also been observed in the past that, in general, when dark witches and wizards attempt to produce a Patronus, their wands will instead produce maggots, which will then turn on the dark witch or wizard and devour them. Except for Severus Snape, none of the Death Eaters are known to have ever conjured a Patronus, although it is also possible none have learned the charm because they have no use for it, because the Dementors are allied with the Death Eaters.