Purchase a black bra. For a good belly-dance bra, find a plain, comfortable, underwire bra in your size. Look for one with standard straps that fastens in the back, but make sure it provides good support, no matter your size.
Cut a circle of black fabric to cover the cup of the bra, plus ½ inch of seam allowance.
Lay the fabric over the cups. Smooth it out and then fold the edges over the cup and pin it. Use small stitches to sew on the fabric. You can sew through both sides of the fabric here, as this will be covered by the ribbon. The fabric will still be a little loose on the cup, so make a dart by folding the extra fabric and whip stitching it.
Cut ribbon to cover the top of the cups. Sew it onto each side of the bra. Depending on the size of the cup, you may need to make a straight dart here, as well. Take a large bead or button and sew it in the middle of the bra. Add your coins or beads wherever you like. You can cover the whole bra or just trim the edges.
Try on the bra and test a few dance moves with it on to make sure it's still comfortable and fits well and that there's nothing to fall off or injure you while dancing. Add more embellishments as desired.