The real trick to doing wingtips is to remember that this stuff STRETCHES as well as shrinks.

Imagine that you have a thin sheet of rubber (Like what a balloon is made of), and you lay it on the wing, then stretch it over the wingtip to make a smooth finish. That's basically what you want to do with your covering, but a heat gun and a few extra inches of covering are needed to hold on to.

PS Wear gloves! Those guns get HOT!