First thing to do is sculpt the masks. For Robin's mask and Spiderman's eyes make a paper template. Next roll the Sculpey out into a thin sheet on a very smooth surface using a pvc pipe or rolling pin. A piece of glass makes a great surface. Use the template to cut out the masks using a razor knife. For Batman's mask just make a large rectangle; no template needed. Place the masks on the gnome faces and pressed them tightly into place. For Batman, drape the rectangular sheet of Sculpey over the gnomes face and cut away the excess. Pinch the nose between your thumb and index finger to create the pointed nose. Use your pinky to make the indentations for the eyes.
Make a little ball of Sculpey and squish it into a circle. Use it to decorate Robin's belt buckle.
For Batman's ears roll two Sculpey balls and shape them into pointy cones. Use your thumb to smoosh them into place on the side of the gnome's head. They'll probably stick there in place and hold on their own.
You should have excess Sculpey rolled out. You can use a razor knife to cut out a Bat-a-rang. Make a paper template if needed.