If you want to sew your own set, you will need:
about 1m of 150cm wide oilcloth (I used tablecloth fabric, available in Spotlight's kitchen section),
about 4m bias binding,
a leather needle (a denim needle would probably be ok; just remember you'll stitch through up to 3 layers of that cloth).
I made the chair bag a bit narrower than the ones they sell, because I noticed the ones in class were too wide and drooped quite a bit. My measurements were: a 40cm square for the main part, two 20x40cm rectangles, one 30x40cm rectangle. To make working with the bias binding easier, I folded it in half and ironed it before I started.
Cut out your pieces. Sew bias binding onto one 40cm edge of each of the rectangular pieces. Position pieces like in the photos, making sure the edges finished with bias binding are facing upwards (for the back) and down (for the front of the chair). Now pin it all together and sew bias binding right around the whole 40x40 square. The bags were just as easy. The library bag's finished measurements are 36x33 with a 10cm flap which I cut on a curve, the smaller book bag is 27x30cm with a 10cm flap. It closes with velcro and I even added some plastic name pouches. Remember to sew the velcro and name pouch before you sew the whole thing together with bias binding. Hope this makes sense; I don't want to teach you how to suck eggs, but let me know in case you would like more specific instructions.