There are a number of methods, depending on your style.

I have used the monokote trim-solvent in the past with great succes.

If you are laying covering over covering, as in a a large area, use a top-flite woodpecker tool to perforate the covering you will be laying on top of (open fram or solid wood below), then use your iron at the low tempurature range for your covering to iron it down, but without causing shrinkage.

If you are overlapping colors, so there essentially one layer of covering, overlap about a quarter inch, there is no need to perforate the lower layer using a tool.

Last time I overlapped colors, I used the trimsolvent to activate the area where the overlap occurred, then ironed the whole thing to the plane.

I would try a few methods on some practice materials to find what method you like best.