Soak them long enough that they will cleanly come off of the backing.
Slide the decal partway off of the backing, put the part hanging off the backing on the rocket, maybe even hold it down with a finger while you slide out the backing. If it's a big decal I will try to wet the surface of the rocket first. After the decal is on the rocket I might put a couple of drops of water on top of the decal to keep it wet while positioning it. The decal needs to be able to slide easily on the surface then you carefully move the decal to where you want it. Then do not touch it again until it dries. I put decals on my Estes Goblin a few days ago, it has several decals on the tube and decals on both sides of 3 of the fins. I had to do this a few decals at a time then let them dry before I did the next few just so I wouldn't risk disturbing what had already been put down.
