I've installed decals on several of my pins and it's relatively easy if you go slow. You do not need to remove the playfield. It makes it a whole lot easier to put the decal on buuuuuut you just removed your whole playfield... If you're not doing a restoration leave the playfield in.
The general technique I use is to put the playfield in a vertical position and push it to one side so you have maximum clearance. Now remove the backing from the decal and hold it with two hands near each corner and keep it stretched out. Now the tricky part is getting the decal to slide into position. It takes some practice and its best to go slow but eventually you'll get it into position. Try and slip it between the vertical playfield and the cabinet without having it get stuck. Line it up and then you just push it on hard and push out the air bubbles. If you realize it's going on poorly, just reverse your steps and try again. The decals are pretty tough and can survive a few glitched applications. Depending on the decal you may have to cut a whole for the hinge bolt. I just do that carefully with an exacto.