J.S.
Depending on how tall or heavy your daughter is, get a green pillowcase standard queen or king. Cut a V in the closed side for a neck hole. Cut arm holes on the side seam. You can cut the bottom like this vvvvvv or leave it the way it is. She will need a belt (wide I think) and buckle (felt is easy to work with, you can even use a hot glue gun)
p.s. pillowcases usually come in packages of two, so don't worry if you think you messed up the first one. :)
Good luck!