This was going to be a comment on plainclothes' answer emphasising something he implied, but I ran out of space...
When making all these notes and scribbles, it's really important to never be afraid of them looking bad. Never be frustrated that they don't come close to living up to the image in your head first time. They won't. Their role is to be a reminder of some element of the complete picture in your head - if they work as a reminder, they're doing their job just fine.
Being able to make sketches and scribbles first time that just are what you picture in your head is an insanely rare amazing talent. Stephen Wiltshire's pretty much the only person alive today I'm aware of who can just do it just like that, and he's a very unusual remarkable person. In general, people who are good at this skill of just doing it near-perfectly first time are more likely to be artworkers or artists than designers: masters of some specific medium or process, rather than designers who need to be able to adapt to new styles and media quickly. They also tend to be conservative about stepping out of this comfort zone.
Of course, the better and more fluent the sketches are the more useful they'll be - but never let it be frustrating and never let it hold you back from an ambitious idea. Your sketchbook (or digital equivalent) is your personal space where anything goes and the only requirements are that it makes sense to you (sometimes, not even that), and helps thoughts and ideas to develop.
Excerpt from the book Sketchbooks: The Hidden Art of Designers, Illustrators, and Creatives. The book gets some bad reviews because so many of the sketches featured really aren't anything special to look at: but that's the whole point. Sketchbooks aren't about creating impressive finished drawings on the pages of the sketchbook. They're a tool to aid the first steps of developing ideas. If it also looks impressive on the pages of your sketchbook, that's a bonus (or, sometimes, a sign that you got carried away and spent longer than needed on that one sketch...).
Once you've got enough details, prompts and reminders down that you won't forget that initial idea, you can take as long as you like (depending on deadlines, budget etc...) to actually create it piece by piece.
It's important and liberating to become comfortable with this - that you can take ideas, stop them fading, and then take as long as it takes to make them real.
If you don't become comfortable with this, it can be crippling. There are months I look back on as being largely wasted because I was reluctant to take risks, allowing good but difficult ideas to slide rather that finding ways to hold them still while I figured out how to make them work.