How to measure roof rafters including the pitch and bird's mouth (seat) cuts (captioned)

Once you know the length of the rafter, how do you correctly mark the angle to cut where the rafter meets the ridge (top) board? And what about where the rafter rests on the outside wall's top plate - how to calculate that seat cut, aka bird's mouth cut?
This is the last of 4 videos in this series.

Видео How to measure roof rafters including the pitch and bird's mouth (seat) cuts (captioned) канала ACCBCT