Chain 90-110 stitches. On mine, there are 90 stitches.
Just keep doing a single stitch back and forth for however many rows you would like. I did 40 rows, but for some people it's the bigger the better :)
Once you've finished, cast off. Then fold it in the shape of the satchel. Slip stitch the sides of the bag together.
Now that you've finished getting it all together, it's time to make the strap. Chain however many you want. It depends on the length you want it to be. On my bag, I chained 140. You have to do 6 Rows of that.
When you're done, cast off again. Attach the strap to the bag however you would like, I just kind of sewed it on with the yarn.
Once you've attached it, pull of the loose ends to the inside of the bag.
OPTIONAL: Sew a button on the front, right where the end of the flap-like thing ends.
To be able to attach the button to the bag, you can add a little loop of yarn. Make a loop and adjust it to the size of your button. Attach it to your bag, again, however you would like, and presto! You have a new bag :)