Find and purchase a product called "Solveset" pronounced solve-ve-set. Maybe even spelled that way. Anyway go to a big time hobby store that sells a lot of airplane model kits and they will have it. Read the directions on the bottle. It's great stuff and really makes the decal lay down so that the edges disappear.
If you can't find that product then add one drop of dish-washing soap to a shallow dish of warm water. drop the the decal paper in the water it will curl up and just as it starts to uncurl use your finger and dab some of the soapy water on the headstock face where you're going to place the decal then lay the decal paper about where you're placing the decal and push lightly down on top of the decal and slide the decal paper out from underneath the decal proper. You'll have a few seconds to center it where you want it. Then make sure there are no bubbles of air between the decal and the face of the headstock (When chasing the bubbles keep your finger wet) Then blot the water on, and around, the decal trying not to move it and let it dry over night. Shoot clear over it...at least three coats and depending how well the finish lays down you're done. If after three coats you can still see the edge of the decal then shoot three more coats and lightly wet sand with a block and repeat if necessary until you can't see the decal edge.