Step 1: Whatchu Need
A sewing machine
Stretchy fabric (jersey, lycra, spandex, even some looser weave cut on the bias will work)
An old bra (optional but extremely helpful)
Elastic (I used picot edged underwear elastic but anything narrow will work, really.)
Step 2: Chopping Things Up
Now it is time to cut your cup into a pattern. If your bra already has seam lines, its easiest to follow those. Mine didn't have any that weren't purely decorative. A balconette bra, which I made, has two pieces: a rainbow-shaped band on top, and a football-shaped bottom piece. Mine was a C cup ad I cut about a 1 1/2 inch rainbow band. It helps to look up pictures of balconette bras in your size and cut your bra the same way.
Finally, cut the rest up. Cut off one band side and the center connector.