I sewed a coat for me at school and I found out none of my rucksacks fit to it:-D
The bag is made from the leftovers of the coat: duvetine (one colour comes from the horizontal waist part of the coat) and lining.
The front piece is patchwork.
Two slantwise laid squares (should be cut as bias): the dark one under the light one. Fold the corners to the center and attach with one vertical and one horizontal line of stitches. Then fold the diagonal edges back and sew on place. They can be stretched out easily and wont fray out as the squares are bias:-)
That's for one piece. If you want more of them neatly joined (I have four), you must sew the squares together at the sides before folding them. They are sewn together exactly at the place of the fold.