Second you need to be able to get upside down in friendly water, a heated pool without chlorine seems ideal. Next while upside down reach your paddle up to the surface with blade flat and strong arm forward. Then pry yourself up with a sweep, keeping head and shoulders low until the boat's pretty much up (bow will be up more of course) and finally thrust that strong side hip toward the weak side, pulling against the thigh strap to get the final umph over, which is when you'll finally sit up.
I am no expert at this! I am just relaying what worked for us and what you can see is exactly what Tim Johnson is doing in the video.