Video Tutorial

NOTES: From the second round to the end, work all the stitches as single crochet Back loops only. Work in the round

Chain 20
Round 1: 1 increase, 18 stitches, 5 stitches at the corner, 18 stitches, 3 stitches at the other corner
Round 2: 1 stitch, 1 increase, 19 stitches, 1 increase, 1 stitch, 1 increase, 1 stitch, 1 increase, 19 stitches, 1 increase, 1 stitch, 1 increase
Rounds 3- 21: 1 stitch around
Round 22: On the sides, count the 8 stitches of the center and place 2 stitch markers. This is the width of the handles. Continue working the stitches to the first marker, chain 8, skip 8 stitches, continue working the stitches to the other marker and repeat the handle with chain 8, skip 8 stitches and continue working the stitches to the end of the row. 
Round 23: Continue working the stitches, in the chain 8, around the chains,  make 8 single crochet, continue to the other handle and repeat the 8 single crochet around the chains. Finish the round with the stitches normally
Fasten off

To addapt the size of the bag, you can make it bigger or smaller starting with a chain larger or shorter. To make the corner you have to make the same as I make in the video.
You can also make more rows with increases, continue making the increases on the increases of the previous rows. Always the increases on the increases, and the make the number of row without  increases to make the bag larger.
You can also make the handles largers making more chains, and skipping the same number of stitches that chains you have