there's youtube videos available regarding the process, but its pretty much that straight-forward:
cut/weed your decals, and pour-on your resin. the surface tension of the resin keeps it from running-off the side of the vinyl letter (as long as you don't apply too much at once) and then the resin forms a "water droplet" curvature that is taller in the center than at the edges.
thought it was neat on a decal, but when I did it to a 1/2" acrylic letter with a metallic laminate, it came out AMAZING! the doming resin really makes metallics POP!