Start in ERG mode and stay in ERG until you can go no further up the ramp. You then switch to level mode for the heart rate constrained section. There are instructions all the way along in the video.
ERG To start, stay seated and keep those legs pedalling until you can’t go anymore… happy suffering!
And in part 2 it is a nice steady one. Remember it is a novid so have a good playlist to hand to keep you motivated