You just glue it onto the hook, paint it, coat it, then tie some stuff behind it. Except for a few details, that's about all there is to it. Pencil poppers, as the name suggests, are long and thin and have a flat face. Cupped face poppers are used to kick up water and make a ton of noise to attract fish in rough, deep, or discolored water. Pencil poppers are better suited for calmer and clearer waters were the loud sounds of a cupped face can spook the fish.
Sliders are particularly effective on smallmouth bass.
Pencil popper - no reason foolin' too much around. Hard foam isn't the only kind of hard headed popper. Cork and Balsa wood are also commonly used. Although you can find some cork shaped to the typical cupped popper shape, more often than not you will have to shape the raw material yourself. Cork floats and casts well, but it's sort of a pain to work with.