As Joe3334 said depends if it's waterslide or sticker.
For starters with a waterslide decal you need to seal it with clear before you use it otherwise the ink will run off the decal when you soak it in water. Then you really want to seal it on the head after you apply it otherwise it can come off or get scratched easily when you're re-stringing. Apply it after wet sanding but don't polish until the very end when everything is finished.
Also it can be a pain but worth the effort doing multiple coats and sanding and repeating over and over until you can't see any edge around the decal. That's if you want a pro factory-like finish. Otherwise it just looks like a sticker.
It's easier to blend the edge if you're using an enamel like Cabot's but not as easy with acrylic lacquer. Whichever clear you use just remember not to mix them unless you want a wrinkle finish like the dashboard on a classic car.

If it's a sticker that's dead easy. Just put it on at the very end after polishing.