There's definitely a million different ways people prepare themselves, you just gotta find what works best for you! However, there are different things you can try - so I'll share my personal experience here haha, hopefully it'll be useful for someone?
For my comic I usually have a scene/character/rough idea to start, so my advice is under the impression you've got this much going. Now I went through lots of different methods while fleshing out my story so I'll just list them at random:
Follow a plot diagram. Like the weird mountain looking thing they make you learn in school. I try to figure out where the ideas/scenes I have can be put on this. (i.e inciting incident, rising action, climax, etc) and try to begin filling in the blanks.
Write out a list of plot points I have in mind, with a (+) or (-) in front, signifying if the event has a positive or negative effect on the protagonist. I tried to keep in mind that my character will struggle more and more towards their goal, so the payoff of completion is more rewarding for them and the reader. This doesn't only apply to physical "real world" challenges like a dragon guarding something precious, but for my comic (a romance) it as more about emotional struggles and the relationships between the cast. It also helped me fill in the blanks on spots where it was getting so sad, its a romance/drama - I need some fluff and stuff too 
*edit: THis can also be useful in figuring out how one event may lead or result in another. Like consequences for certain actions n such.
I list out my characters goals and motivations. It makes me think deeper on what they're trying to accomplish in the story and how they might get there. Not only the protag, but any characters of significance.
Sometimes when working on characters I like to look at zodiac sites. It lists various weakness and strengths people have that I might not normally think of. Sometimes I try to see if the idea for a character falls under one of the zodiac personalities, and chose their birthday that way for fun ^^
Aah this is getting long sorry! But those are various things I try while developing a project. As for how much I have planned - I have the big points of my plot and some smaller stuff planned out on one big outline. And a more in depth episode by episode script done up to about three episodes further than where I currently am. (THo I'd prefer to be a bit further ahead in this personally)
It depends tho, some people do better having more planned. Some find it sucks the fun and inspiration out of it. Whatever works for you ^^ Goodluck!