My game design notes have improved a ton ever since I started taking a bit of time to go back and edit/organize them with section titles for each idea, and bold text for a few keywords like NPC dialogue in a Zelda game. Makes them much easier to come back to.

Also a picture really is worth a thousand words so doodles, sketches, references, screenshots, etc. go a long way to solidify an idea enough that you’ll actually remember exactly what your thought was when you come back to it in 2 months.