I figured pirates are so boring, so I went for mermaid.
I used an old bra that I didn't like wearing because the one of the straps broke.
Start by turning the bra inside out. Then, with a pencil i drew on the shape of a shell on the inside of both cups.
I struggled a little to get it symmetrical.
Then cut the bra along the top ridge of the shell.
Turn the bra back the right way around and with a permanent marker, draw the shell shape on the front of the bra.
I then cut both the straps loose from the discard-able top of the cups and sew them (by hand) onto the 'new' cups.
Sure, it isn't a great bra to go jogging in. (the cups are only half their original size)
But it will work BEAUTIFULLY for my party