There's no definitive way to start a micronation, but this article shows you some methods.
There is no need to rush this process! Take a look at other micronations, see what you like and what you don't like, and take inspiration. Create drafts of names and flags to see what is best. Nobody is rushing you, so take your time.
Unless you're thinking about starting an online entity, like a network state, you'll need some land. You won't own this land, nor will you have to buy it - you can create a micronation outside of your property such as a park or beach.
However, making a presence in a public area might be difficult. You still have to follow laws, so planting flags and creating buildings is generally illegal. Therefore, many micronations are set up in one's property, like your garden or a room in your house. Here, you have freedom to decorate!
Think about your micronation's name and flag. These will be the first impression people have when they see your micronation. So, make sure they are memorable and represent your micronation effectively.
This identity will stick, so make sure you're happy with it before commiting! For inspiration, check out the MicroWiki's extensive list of micronations.
You could even go on to create your own branding pallete, pick an anthem or develop a currency. Go wild!
Your micronation shouldn't be a lawless land; think about forming a constitution and rulebook that your citizens should abide by. Make sure that these laws also incorporate those of the real government: for example, murder cannot be made legal.
Note: In some countries, enforcing your own laws could risk prosecution. Always check first.
Make yourself known to people by joining communities such as r/micronations and joining micronation organisations such as the United Micronations Union. They're a great place to meet other micronations and form bonds. That's how the UMU grew, and that's how you can too!
Another way to spread awareness of your micronations is running events. The more participants, the more will know about you. Things like chess tournaments, song contests and e-sports will be popular, especially when others get to back their micronation.
You can create a professional website using tools such as Google Sites; it's free, intuitive and powerful. This website was made with it!