Brastructable.

Let's make a bra! I love making my own lingerie, and this is a fun and semi-supportive bra that is super customizable. I made mine out of stretch lace and t-shirt scraps, but you can make it out of anything stretchy that you want!

You, Bra Creator, need these things:
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.)

Time to pull apart your old bra! First, cut the cup off under the underwire channel. Then, cut out the underwires, and save them if you want to put them in your new bra (I chose not to). You can also pull out the little plastic side pieces if you want the sides of your bra to have extra structure and add those to your new one. Finally, cut off the hook and eye tabs, leaving a quarter inch seam allowance, and save for later.
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. 

Your bra is cut into a pattern (Or you just made something up, if you didn't feel like cutting up a bra) and now you need to trace out the pieces! Trace the two cup pieces onto pattern paper, adding seam allowance in between the two. Then, trace the side band, stretching any elastic but NOT the fabric. Just try to lay it flat and trace it. Do the center connector, getting the angle as accurate as possible. Add hem allowance on the top of the center bit. Remember that you need two of everything except the center piece, so write it on your pattern pieces to remind yourself!

For my bra, I wanted the top of my balconette to be sheer. I decided to use a strip of stretch lace. Since I wanted to keep both scalloped edges intact, and I knew that the stretch in the fabric would make up for small changes in shape, I decided not to cut it exactly to the pattern piece's shape. I made the edges at the right angles and tried to match the length. 
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.

Sew that bad boy together. Remember to refer back to the probably-still-intact other half of your old bra for placement of things and whatnot. If you are a seasoned seamster, continue on and ignore the following directions. If you are a beginner, let me help you out:
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. 

To measure how much elastic you need, wrap it around where the band will sit (right under your bust) and make it snug but not pinching. Cut that length. Mark the center point of your elastic and your bra, pin it there, and stretch appropriately to make the elastic the same length as the bra as you sew. Use a zig-zag stitch!!! Try to sew closer to the top edge of the elastic so it doesn't make weird folds when you wear it. 
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. 

Okay, super seamster, now just throw on those hook and eye tabs you chopped off your old bra. I sewed them right sides together and the topstitched the seam allowance to one side so it wouldn't poke out when I wear it. If you are a super dedicated person to making everything from scratch, you can make your own tabs and sew individual hooks and eyes to them, but that will take like 45 hours and your fingers will hurt so much afterwards. 
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. 

I decided not to add any embellishments since I had so many crazy fabrics going on, but feel free to add a sweet bow or button to the middle, on the ugly spots you want to cover, or any other place. 
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!