You don't have to do anything special. This is one of those really cool things that works like magic.

Just draw a line (Lengthwise) down the center of one of the two sides that meet at a 90 degree angle.

Draw a line lengthwise through the center of the last rib.

Now, glue the wingtip to the end rib keeping the lines even

Now just sand the block to the same shape as the wing with a sanding bar (You may want to shave some excess off with a knife first - and places where you dig too deep can be filled later)

Placing Duct Tape on the wing will protect the sheeting while you are doing most of the rough sanding. Remove the tape when you're sanding more tape than tip.