Every year, for the past several years, my mom has done the Susan G. Komen Race for the Cure, here in DC. It is a 5k walk or run and she does the walk part. I have gone with her a few times and my sisters have gone a few times. Anyway, every year, they send you a shirt after you sign up. I don't know what's become of the other shirts I got, but this year I decided to try to do something with it that would make it more wearable.
Here's a photo of the shirt before any alterations:
The finished shirt, front:
And the racer back:
Step 1) Wash your shirt! Free t-shirts are typically not pre-shrunk, and it would be sad for you do refashion it into something that doesn't fit!
Step 2) While your shirt is in the wash, find a tank that you want to copy to make a pattern. Any large paper will work for the pattern. Tissue paper left over from something you bought at Anthro, an unfolded paper bag, newspaper, old wrapping paper you've saved for some reason... you get the idea.
I traced a (Splendid) tank I have that fits me well. It's one of those sort of flow-y styles. It's slightly fitted at the top and then a-line, loose at the bottom. Here's a drawing of the basic shape of the tank. To fit the straight sides of the t-shirts, i just made the sides straight. The dotted line shows the way the Splendid tank is more a-line shaped off of the front piece. It has a very slight curve to it. I didn't actually digitally trace the pattern I drew out, so these are just rough shapes. If you want to make one yourself, you should definitely trace an article of clothing of your own. If you don't have a racer back tank, you could use a sports bra with a regular tank top!
To make the tracing easier, I flipped the tank inside out and then folded it in half. To trace the front, hide as much of the back as possible, while keeping the front part as wrinkle free as possible. You can trace it right up against the edge for the neck and arm holes. Unless the shirt is really too big for you, you should not need to cut the sides at all. The shape of the back is funky because I did some shirring/faux smocking in the racer back area.
Step 3) Trace your pattern onto your shirt. If you use a disappearing ink pen, you can do this on the right side of the shirt to make sure you capture any designs printed. If you either don't care about the pattern or are confident that it won't get cut off, it is somewhat easier to trace if the shirt is inside out. To make this easier, I flipped the shirt inside out and folded it in half. I ironed the fold crease just to make it more defined.
**Be sure to do this with the shirt inside out or you will melt the screen print!
Then you can go ahead and place your pattern and trace. When you trace, it is likely that your tank's arm holes will be smaller than the opening of the sleeve. That's ok.
Step 4) Cut out your tank. I like to start with the front, because the back can withstand a little more fudging if you mess up or if your tracing wasn't quite exact. (Almost nothing I do is exact!) Carefully make a small (0.25" worked for me) cut along the line you drew up at the shoulder seam. Insert your scissors here so that you can leave the shoulder seam intact for your tank. When you reach the sleeve, cut against the seam, leaving the seam attached to the sleeve and cut all the way down to the armpit. When you've cut out the whole front, flip the shirt over and have a look at where the shoulders are. Hopefully they match where you've drawn the shoulder from the pattern. If not, re-draw so that it matches up! Remember that you want the middle racer back area to be fairly wide. Mine was about twice as wide as I wanted it to end up.
Step 5) Shirr!!! I used this tutorial. You can draw lines along the area if you want. I didn't, because I don't do exact sewing! I do between 5 and 8 rows of shirring. At the end of each row, I knotted both ends before cutting off the excess. This allowed me to control the amount of gather and elasticity.
Step 6) Dig through your stash of bias binding and pick one. Before you start binding, you will need to sew up the little seams at the arm pits. This is a good time to try on your tank. You can make these mini-seams more like darts and pull in the shirt more if it's too big. When those seams are sewn up, I knot the thread ends also (instead of back tacking, I think it makes it a little neater because the excess is cut off and hidden in the bias binding. I alternately fudge and sew bias binding properly. Prudent Baby has a good tutorial (and lots of other helpful stuff).
Step 7) Trim any loose threads and put it on! You are now the proud owner of a fantastic new, perfectly wearable tank top!