Whenever a random idea pops into my head I instantly write it or type it down either in my Grammarly or google docs or whatever I can get my hands on, a napkin, etc. It’s one of the many reasons why I carry around a sticky note or small handbook with me. 😊 so I can come back later to fix it and flush it out more. I have a doc each with my ideas. One for world-building, one for char designs, another for ideas I could use for said designs, and plot ideas. I can not stress how much I spend the whole day trying to remember the idea I had last night after telling myself I’ll remember it in the morning.