I am covering a LT 40 for a friend actualy

for the wings I use a two color scheme, the color transition is done at the main spar.
I don't recomend you to seal your two covering pieces together before aplying on the plane.

. first apply the lightest color, then the darkest one and use the all surface of the spar for the seal.
cover the the two wingtips first (like the book call for)
when I have bubles i simply use a pin to make a hole in the buble and then i iron the area

my suggestion is to first iron the bottom of the wing and then the top.