Do you have a smartphone? Just hit the voice command button and say "Make a note: [whatever you need to make a note of]"

Alternatively, I find it's easier to do things when you've already started. Maybe make a note on your phone and add a couple ideas. Then, the next time you have an idea, you already have a dedicated place to put it, so you just put it there instead of wondering what to do with it and ending up doing nothing because you can't get past the inertia of starting. Then award yourself a mental point as a reward to train your mind to keep coming up with ideas and writing them down.

And if you do forget something, don't worry about it too much. The whole "I just had a really good idea but now I forgot, oh no!" pattern is really common. Much more common than the "I just had a great idea and I wrote it down and I still think it's great" pattern. I've always taken this as evidence that the idea you forgot wasn't actually that great and it only feels that way because you forgot about it and you're suffering from a kind of forbidden-fruit/grass-is-greener bias. People tend to remember really good ideas because they're contextual or actionable.