Idea appears suddenly, at its own schedule, but to have more ideas in general, it might be good to do your visual research for a bit every day.
I never write the idea down because it should be recurrent or exciting enough to be remembered otherwise I won’t have enough motivation to spend time working on it.
In the first moment the idea is very vague and gives only a starting point, not the whole story. I like to give it at least a week or two of background developing before I start consciously working on it.
Example: I know that I will travel to Kiev and I want to make a photoshoot based on my childhood memory of 1990th