Advice for a novice getting ready to put in his first full days work

TL;DR ADHD looking for tips on how to structure my time to efficiently transition from brainstorming to writing

I've been writing as a hobby for a few weeks now, I feel strong on prose once I have an idea about the overall structure of what I want to write. Where I struggle is once I have a character idea I find compelling, how to get to the point where I transition from brainstorming to writing prose. I'll come up with the character, have an idea of what I would like them to be dealing with in a specific chapter or short story, but struggle to figure out the specifics of it and I'll just write an entertaining intro but not know how to get them to the next part of their adventure.

Do you guys dedicate chunks of time to character/world building/plot development, then just force yourself to get started on the first draft? Being ADHD, it's generally a bad idea for me to just say "ok I'll just write for 2 hours now". I need a sense of direction to start out or I just end up brainstorming small bits of many different small threads