If you know how to make a basic granny square you don't really need a pattern to make this bag, but I have written out what I did anyway. If you don't know how to make a granny square, you'll find the directions here.
Repeat the 5th round 8 more times (the "wall" is 9 rows high)
On the 14th round, chain 1 between the blocks instead of 2
On the 15th round, make 2 dc blocks instead of 3 dc, chain 1 between all blocks
On the 16th round, make 3 dc blocks instead of 2 dc, chain 1 between all blocks
On the 17th and last round, chain 2 between all blocks instead of 1
Twist a cord of a single strand of yarn and weave it through the 15th round.