About the only way to hand-do scallops is to use a round punch with a flat, or slightly concave face.
Hold the punch AGAINST the surface at an angle, and strike the punch.
This will leave a semi-circular imprint on the surface.
Move the punch over so the edges will touch and strike again.
The next row, shift the punch over 1/2 so the rows form a fish scale pattern.
You'll have to keep the edge of the punch sharp.