I've made a few of these already and I must say I am in love with them. They are comfortable and it's not to much fabric for when you're working out. It's best if you use a shirt thats a size to big for you otherwise it might get a little tight.
Get an old tshirt and cut the sleeves off at the seams.
Next, cut off the collar.
Then cut about 1.5 inches farther down into the collar. When you get halfway fold it over so you can use it as a template so the collar of the tank top won't be crooked.
Start about an inch away from the edge of the collar and cut all the way to the bottom of the arm holes.
Now cut about a 2 inch strip from one of the sleeves and cut it so it lays flat.
Bunch up the back of the tshirt and wrap the strip around it twice and tie a knot.
You can twist the strip so that the know is in the inside.
Now you have an awesome new tank top :)