Andy,
I hate to state the obvious, but slow and steady. You can always remove material, but it is a real pain to fill in a gouge. Try a chisel xacto blade and file the corners round, that way you won't dig in to the plastic.Attach, with white glue, pieces of sanding film to pieces of styrofoam egg cartons cut to the shape you might need for removing material. If you do gouge, make a slurry of you favorite filler, fill the scrape and let it dry overnight. Then try again.
HTH