Film scoring is not about knowing how to sound like John Williams, it is about understanding how the music of JW helps the story at some given point in time.
1.An easy and effective exercise that you can do a lot is to select a moment you like in some film production and try to fit some already composed music of other composer to it. Try this with different composers and pieces for the same scene. Observe and write down some analysis of the outcome.
2. You can also do this with images.Images have the advantage of capturing the essence of some message. They will relate easier to the piece of music because they are static. Try to find some piece of the impressionist movement that fits to this picture, for example:
This way your are training the most essential skill of the incidental composer: the ability to relate musical events to events that are external to music by perceiving the analogical bond between both and developing your sensibility for this perception.