Make sure your disconnected from shore power/Gen is off.
Just remove the cover.
Install new 20/20 tandem breaker. It just sits on the bottom rail at a 45 degree angle, then push in on the top until is snaps into place.
Move the black wire from the old 20 amp pole (mine was for the AC unit) over to the new CB.
On the breaker cover, you can snap out the new circuit breaker spot that your going to use. Reinstall cover and your good to go.
You'll notice there is one more free spot to use if needed in the future. And actually, there is only 2 free slots prior to this install, FYI. Very simple task. Just sucks laying down on the floor under the foot of the bed. Then again, ours is a California King foam mattress, so it sticks out a little more than normal.