My usual approach is to work a curved belt buckle from the back of a buckle using a curved nylon or rawhide mallet. Ponding the metal into a leather bag filled with lead shot or a 10 lb chunk of plumbers lead I melted in a small cast iron skillet.
I also have a hardwood chopping bowl my sister, the gourmet cook, gave me that works too. Also, you can use a piece of a 2x4 construction lumber.
Also, you can use an anvil and move the metal in small increments while hammering from the back.
Be sure to anneal the metal often to ease the bending and prevent cracking.