First, cut the bra up. Basically, you're cutting the cups out. Cut close to the underwire, but don't cut the fabric around the underwire open or it will end up poking out. Save the scraps for later.
Use hot glue to attach the cups. If you use too much, it'll ooze out the bottom – so be careful.
At this point, if you don't like how the bottom of the purse looks, cover it with some ribbon, beads, or fringe.
Next, you're going to add the straps. This bra only had one short strap (because it was a halter-style), so I made it like a little handbag instead of a purse with long straps. If your bra doesn't have long enough straps, you can use yarn or ribbon to make them (or lengthen them).
Add embellishments to the outside of your purse, if you'd like. I think buttons and beads are cute. You can sew them on, but hot glue should be fine