Sometimes, I have no idea where to start a scene. Don’t let that stop you from writing the scene! Don’t forget the seat-in-the-chair rule 🙂
I usually put a comment at the beginning of the scene, giving myself ideas about where to start. Then I plunge into the scene ~ just write. By the time I have the scene written, I can usually go back to the top and find the appropriate place to start, not too early, not too late.
After that, I add the description necessary to ground the scene for the reader. Enjoy!