It's very important that you don't judge your own ideas as you have them. Instead of "Oh, that'll never work, I won't write it down", think "This might not work, but I'll write it down because it could lead to other ideas that work very well."
Judgement and self-doubt are paralysing, and can stop creative thinking very quickly. You should feel comfortable to write down the worst ideas you've ever come up with as well as the ones you think are the best. Every idea can lead to many more when you are able to consider their merits.
Writing down ideas even when they're "bad" can also help to warm up your creativity and get you into the rhythm of coming up with ideas, which means you'll come up with more ideas, more easily. As explained elsewhere, that's the best way to come up with good ideas.Mechanics over inspiration → ← Impossible is not a problem Back to the table of contents