I think the most important thing mentioned in the script and video is to recognise that when your mind wanders, just let it, go have a break.
When you come back to the project, you find new insight, sometimes an original plan will have changed so you produce something better, a new idea surfaces so you can add that to the project, A seemingly difficult part turns out to be quite easy, and so on.
Whether writing new music, or building a garden shed from scratch, it doesn't matter what, just take that break, which gives your mind a chance to work in the subconscious realms.