My first undercambered glider wing was covered with monokote.

Start at the high point of the undercamber, tack the covering down along each rib to the leading edge. Get all the ribs done, get the leading edge done, but don't shrink it . Now, tack along each rib to the trailing edge, tack it all down along the trailing edge and when you have the whole thing done, then shrink the whole wing. Be careful to not let any of the covering pull up from the ribs. If it pulls away, stop what your doing and tack it back down.

That's for film...follow the same advice for whatever covering you decide to use. Just start at the high point of the undercamber, work forward, then work back, then shrink.

Don