What you need:
Fabric crayons (optional)
1. You have tee shirt that fits you a little loose and it's pretty boring looking. (mine is a vintage disneyland tee)
2. You take the shirt and fold it in half.
3. You then cut the sleeves and a little more until you have an idea of the desired width of the straps to the shirt that you want; although you should save some room to cut the collar off.
4. While the shirt is still folded in half, cut the collar off.
5. Unfold the shirt and cut off the remaining part of the collar.
6. You then cut a rectangle; the width depending on how thin you want the straps; on the front of shirt you can cut down to however lowcut the shirt you want.
7. Then cut a medium or small half circle leaving a strap in the middle of the back of the shirt. Then cut the strap in half.
8. Then tie both of the half strip together and you're finished.
For another t-shrit reconstruction idea, you can watch this video (that I made a long time ago) :)