My tailgate decal was applied with a mixture of (I think) water and metho. My local sign guy did it in about 15 mins and removed all water mix and air bubbles with a squeegee. The advantage of using the water is so you can slide the decal into place for perfect positioning.

Before he did the tailgate, I have always applied decals dry, but you need to be careful to get them in the right place because they look stupid if they aren't straight (or have a load of bubbles trapped beneath).