Turn the T-shirt inside out and fold it in half lengthwise so the sleeves are on top of each other.
Using sharp scissors cut the sleeves off of the T-shirt, then cut off the neck.
Line up the armhole handles so that the side seam of the T-shirt is now in the middle of the bag. If your T-shirt has a design on the front you can instead lay the t-shirt flat so the seams are both on the sides.
Sewn T-shirt Bag
Sew the bottom of the T-shirt closed using the zig-zag stitch on your sewing machine or with a needle and thread. You can also use a serger if you have one.
No Sew T-shirt Bag
If you don't want to sew the bottom, then place some pins along the bottom edge of the shirt about 4 inches above the hem. Pin all the way across.
Use sharp scissors to cut vertical slits across the bottom of the shirt.
Take one cut strip from the front of the shirt and the one directly behind it and tie them together. Continue tieing until you've tied all of the cut pieces and closed the bottom of the bag.
Enjoy your new T-shirt bag! And maybe make another one to go with it.
Notes
If you want a drawstring bottom bag, see the directions in the article above.