First, use a piece of vellum (or another scrap of paper) as a template - trace the inside of the belt buckle with your pencil. I did a rubbing of the edge and it worked well.
Tape the template on your paper and cut it out (with scissors or a craft knife).
Mod Podge the paper into the buckle blank and let it dry for a few hours.
Make sure to let the paper completely dry - if there is moisture under the Dimensional Magic, the entire project can dry cloudy.
Add embellishments (you can't see them under my hand in this photo, ha!) and then add Dimensional Magic. Here's the whole trick to the belt buckles: THREE layers.I did one layer and allowed to dry in a warm place (to prevent cracking). I did a second layer and allowed to dry again. THEN I did a third layer and added the scrapbooking brad embellishments so that they would be "dimensional" on the top of the Dimensional Magic.
Let dry overnight, preferably for 24 hours, before attaching to a leather belt.