Personally, I use Blake Snyder's Save the Cat beat sheet, which has 15 plot points to lay out the story. Whenever that initial idea for a great movie hits me, I take the beat sheet and address each beat with a few sentences. If at the end it feels strong, then I go into prep (underlying theme, character backstories, what happened before start credits, settings research, dialogue research, the corkboard, etc.) but that beat sheet keeps me on track. Eventually I will lose some of the beats mentioned in the sheet and keep the main ones, but the list keeps, and later on the corckboard, help me stay focused and get to the end. I strongly recommend it.