I don't claim to be expert at it, but I have found that I get the best results by giving them plenty of soaking time in the water, so they float off the paper easily.
The biggest problems come when the decal sticks to the paper in places, and doesn't let go.
My method is soak it till it feels like it moves on the paper.
Then bring the paper with decal on it over to the bike, and then position the paper right up against the tank, and slide the decal off onto the tank. You have a little time while it is still wet to do final positioning adjustments.
Use your finger or a wet sponge to squeeze any air bubbles out the edges. Usually I don't get any air bubbles, but it sometimes might get under a large decal.
Handle with care.