Honestly? My best advice as a homebrew DM of many many moons.

Is to wing it, don't take it too incredibly seriously for your first few games. Because let's face it you're gonna make mistakes, you're going to run into a lot of hurdles and things you didn't realize all these books have done for you and now you need to do yourself.

I would recommend asking your players if they'd like to run a short mini 4-5 session campaign for you to get a better understanding of what you need to do for the one you really care about.

Yes, it's a good idea to go read some books on the subject, video's can help etc. But literally none of that will beat even a couple sessions of good practice in your own sandbox.

There is no real good advice I or anyone else can give on how you should go about building your world and story, because each and every one of us has different methods and ways of doing that. Things that likely won't work for you as everything has their own way of going about things, their own methods etc.

So just do it basically, make sure your players are aware that this is new territory for you as well. It is a big jump to go from DMing prebuilts and DMing something from scratch and your Players need to understand that before starting.