I used an old tee that doesn't fit me anymore...
1
Cut off the sleeves, and if you want, the two bottom corners.
2
Sew the borders...
3
[This step is optional]
Cut a cloth that's double the size of one face of the bag.
4
Sew it like you did with the tee.
5
Put it inside the bag and sew it to the bag.
6
Choose two cloth stripes sew them as you want.
7
Your bag is ready!!! :D