First you get your ideas down on paper on in a computer to wherever you jot things down that you don’t want to forget. That way, you can stop worrying that you’ll forget them because they are written down. Somewhere.
Second you organize those ideas using whatever system you prefer. Most important to least. Chronological. Most favorite to least favorite. Whatever works for you.
Third you put them aside and, now that they are in a place where they won’t be forgotten but can’t bother you any longer and keep you from thinking of your current work, you figure out what you DO want to do. What you DO want to write.
Fourth you outline that wonderful, creative work so you’ll have some idea of how you want to put it down on paper because, if your mind is the kind of mind that gets lost in a blizzard of ideas, then you’ll need to organize physically, on paper, in order to not have those new ideas start haunting you just like the old ones did and preventing you from writing.
Fifth you write.