I always start thinking on paper, like the quick sketch above. These quick doodles would take much longer on a computer, and they help to understand the requirements of the final deliverable. In this case, that deliverable is a short video to explain a common UI transition.
As UX owner for Garmin's connected infotainment platform, I am responsible for crafting the UX guidelines for the entire platform. Such a massive spec includes UI building blocks, patterns, page types, etc. I pass these guidelines along to other designers, engineers, and management to help explain how features should work.