Position the positive end pointing up, with a couple mm of wrap extending above the top. Hold the wrap around the battery about halfway down the side as you heat the top. Gravity holds the insulator in place.I used a blow dryer. About burned my finger off trying to keep the insulator on long enough to get that side to flip down. I have a heat gun somewhere. But where is the operative part of it.
If you are still getting very hot fingers you probably need to switch to low power on the dryer. Also, make sure you are moving the dryer back and forth, you don't hold it still at any time.