Self-employment, The ultimate goal for most people.
I managed to accomplish this at the age of 23! I quit my job in August of 2016. I prepared in every way imaginable, Growing several sources of income, And even using my vacation time to "practice" the self-employed life, For three weeks to see if I was capable of handling it.

Your Emotional State

Being self-employed is a little harder than I expected on an emotional level. And It turned me slightly into a hermit. While this may not be the case with a lot of self-employed people. This was the case with me. Being self-employed changes your life in a lot of ways you wouldn't expect it to. And you don't fully see the effects until you become self-employed. Which is something to keep in mind!
I always forget that other people have 9-5 jobs, So It's always bizarre to me seeing people drive to work. Haha. Little things like that make your life truly unique.
Also, it can be a lonely life, If you don't have a lot of friends, Or if you don't surround yourself with people. I Underestimated how happy my Coworkers at Ups made me each day. And this was a hard thing to deal with when I became self-employed.

What Is Your Financial State?

You obviously need to be able to cover your bills once you quit. But you also need to make sure your business or self-employed gig, Will be around for a long period.
You also need to have back up plans in place in case something falls apart, As well as multiple sources of income.
Here's an example of my income sources currently.

1. Steemit (For Emergencies only)
2. My Shirt Business (It's diversified on four platforms)
3. My Coloring Book Sales
4. My Kindle Book Sales
5. My Ecommerce profits that are owed to me the next two years.
6. UPS
   
   Currently, my biggest income sources are my shirt business And second place Steemit. Let's say tomorrow; Amazon bans me. That would cause 1/2 of my Shirt business to be shut down in one day. And that's if Amazon were to ban BOTH my Merch And my Seller Central Account (Usually only one is banned at a time)
   I would obviously be in big trouble.
   But guess what?
   I'd survive.
   Because even though 2 of my ships would have "Sunk." I still have my other sources and platforms to provide me with money.

Can you REPLACE Your Job?

No I'm not saying you should bank on the fact that you'll need your job back. But when I say "replace". Think deeply about the benefits that your job is offering, Can you replace those things?
The good health insurance?
The discounted movie tickets?
Every little thing that you use, You need to keep this in mind.
When I left UPS. I left some of the best health insurance you can have. Plus I was only required to work part-time, And NO I never even had to pay 1.00 out of my paycheck for it.
That's a pretty big sacrifice, The insurance I have now, Sucks. So if I need any health or dental care, I pay a lot out of my pocket.

The Less Obvious Benefits

If you work a labor-intensive job, Going from that, To potentially working a desk/ Self-employed job. Will cause you to gain weight. That's a big reason I went back to UPS. Because I started gaining all this weight, Even though I was running 6 miles per day. It got the point, Just to maintain the weight I was at, I'd have to have an almost perfect diet.
Which was hard.
These benefits can add up; You need to find a replacement.

For me, I could have gotten a gym membership. But why pay money to work out for a few hours every day to keep my weight down. When I can go to UPS and load boxes as fast as I can and keep my weight down that way. Plus get a weekly paycheck.

Becoming self-employed is a wonderful and empowering thing.
But it also can have downsides that many fail to anticipate.
Hopefully, this helps you get a clearer view from other angles so you can prepare better.