what you need: styrofoam skull, knife/box cutter, pre-packaged plant
I bought a styrofoam skull from Michaels, and it was very easy to work with. Trace the outline of your plant's pot and cut out along that outline. Cut deep into the styrofoam so you can leave plenty of room for the pot to set down in.
Add your plant and make sure it has adequate room. Add fun touches like a plant coming out of the eye, teeth, etc. And voila!