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.
Step 3: Making a Pattern
Step 4: Picking a Fabric and Elastic
For my bottom piece, I found a t shirt scrap that was a nice color and fit both pieces; for my connector piece and my bands, I chose a playful scrap of t shirt that I needed to use up anyway. For my edges and my straps, I used bright blue picot edged elastic. Fold-over elastic would look nice here too. You can use all one fabric, you can use all different fabrics; you can make it symmetrical, you can make it out of 7 different colors, whatever! Just try to match the stretch level reasonably well.
Line up your pattern, and cut out the pieces! The elastic will come in later.
Also, look for thread that matches your elastic, ideally. I had green. Close enough.
Step 5: Sewing It Up
I wanted to keep the pretty edges on my stretch lace, so I just top-stitched my cup pieces together, right sides both up. if you aren't particularly attached to any edges, you should place them right sides together and sew so the seam allowance is hidden. Use that second method to sew on the bands to the sides. Now is the time to make underwire and boning channels if you want to use them. To do this, refer to the second picture with the artful pink drawing on it.
Step 6: Elastic Time
The top of the band is a bit trickier. I estimated using the length of the elastic on the bottom band and tried to pull the same level of stretch when I sewed it on. It ended up a bit tight in the armpits, so maybe be a bit gentler with this part.
Step 7: Fastenings and Straps
Straps! Slap that bra on your front and hook it together. Now, safety pin one end of your extra elastic to the front and pull it over your shoulder, tightening until it feels comfy and won't pull anything wonky. Keep your finger there, cut it, and cut another the same length. Super super super sew them onto your bra, you don't want them fraying off in the middle of a class or while you're at work. I totally could have just not cut my top band elastic and continued it as a strap, but i did not think ahead that far. You are welcome to do that if your bra style allows.
Step 8: Embellish and Show Off
I believe that you you are done! Show off that bad boy you worked so hard for. Wear it under a tank top with huge 2 foot long armpit holes just to show the world how crafty you are. Or, keep it hidden as a fun little secret and check yourself out in the mirror.
As a variation, you could make this out of swimsuit fabric for a more structured bikini style, or all sheer fabric for a super sexy style. Also, you could add padding in by simply making a pocket like you did for underwires, but pad-sized. Awesome!