Cut out two pieces of a fabric you like. I chose the back and the front of a skirt made of a quite thick fabric. The shape of my bag looks something like this (not exactly but :P) and the size is around 40x40 cm.
Fold and sew the top of each piece.
Place the pieces inside out and sew them together around the edge.
Attach a strap. For my bag I used the belt from the skirt which is a braid in the same colours as the fabric.