Having made way too many handprint and footprint crafts in C's short 15 months on this earth, I like to think that I have it figured out. I use washable, non-toxic tempura paint (this is Sargent Art brand from Hobby Lobby - $1.59 for a good-sized bottle) and a mini foam craft roller (also from Hobby Lobby) to apply the paint to their little feet. The foam roller results in a nice, even application of paint and gets it done much more quickly than applying with a paint brush (and dipping their feet into a plate of paint makes it way too gloppy). I've used a cheap foam paintbrush too, and it works, but it takes longer and the paint isn't as even.
Other than paint and some way to apply it (foam roller, paintbrush, etc), all you need is white construction paper or cardstock, a black Sharpie, a white paint pen or liquid white-out, and a silver Sharpie! (If you don't have a silver Sharpie and don't want to buy one, you can just do the ear things in black instead of silver).
This craft is pretty easy and you can embellish it however you want -- we chose to write "Trick or Treat" on ours. I've included a printable of the "Trick or Treat" design below - just print it on white letter-sized paper and get going with your Frankenstein footprint-making!