Find a black or pink leotard. Cut a white collar and cuffs off of an old shirt or one you find at the thrift store. Cut carefully so the fabric doesn't fray. If you can find a cheap bow tie, wear it on the collar. Otherwise, make a bow tie out of some black ribbon, simply tied around your collar. Safety pin the front of the collar to keep it on your neck. Button the cuffs at your wrists.
Wear a pair of heels that match your leotard or bunny ears.
The ears and tail:
Here is a tutorial on how to make bunny ears. For the tail, get a bit of pillow stuffing and stretch it, gathering at the end. If you don't have a pillow to mutilate, just buy a cheap bag of pillow stuffing at the craft store. Wrap the gathered end tightly with string, then pin to the back of your leotard.
Now spend the night trying to avoid getting your butt pinched.
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