How do we manage that? Time, patience, and holding on to the concept--at least long enough to write an opening scene with the idea/concept in mind. We might jot down some notes, but we might also go do some research if the idea calls for it.
Working on my latest, I had to determine the best most recent treatment for epilepsy because the answers would support my wild-haired idea for a story involving past life regression meets astral traveler. Heady ideas combining. The research supported my idea, fed it in fact.
I believe in research informing ideas. Through delving deeper into an idea via reading, research, and googling these days, one nurtures the notion(s) as the book concept grows and extends from scent to scene, chapter to chapter.
Even in my lowliest most schlockiest horror novel (novel in novel idea) such as The Sub-TerraneanS or Bayou Wulf, just as in my more serious historical novels (The Red Path, Annie's War) I do research to inform myself of the various aspects of a concept/idea.
Out of the IN basket comes a slew of information then which informs me on how to pull the necessary threads of a novel from page one to page last.