here's what i did.......i actually went farther than i needed to, in that i went well beyond the wing chord. when trimming the excess, i actuall pulled the covering up so that i was cutting the "smooth" part of the covering. after than i just ironed it back down.

you can also try turning your heat up to max BUT you have to move quickly and not stay in any one place or the covering will shring and pull apart at the seam. (been there got the t-shirt)