Step 1: Make your costume
FIRST: get a long white dress for your model. i got one from a thrift store for $3 which I highly recommend if you plan on cutting it up or throwing dirt on it like i did.
NEXT: cut it up and make it look worn & torn. i threw dirt, flour, and red food dye mixed with vinegar and then olive oil on the dress my model wore to make it look authentic and as if she just rose from the dead after a bloody, violent death.
Step 2: Take your photograph
Step 3: Cut out your model
Then you need to select the lasso tool and use it to outline your model. This is how you will cut the person out. Once you have your model selected, or outlined, go to the Select tab then click inverse then delete. this will leave you with only your model. Put the model picture above the background in your layers menu on the bottom right hand side of the screen.