Sometimes I specifically plan to shoot a scene. Other times I simply take advantage of what opportunities present as go about the usual business of life. Driving home the light and clouds catch my attention. I stop along the road, setting up the camera and shooting 20 minutes of time lapse. This will become ten seconds of video, clouds sweeping past the summit of Mauna Kea in late afternoon light.
I have located several more pieces of music appropriate for soundtracks. It is the music I start with, building the video around the soundtrack. With music in hand I can plan, visualizing the finished video, deciding what further scenes I need to go out and shoot.
Keep a camera handy.