Step 1: Sizing!
I took the largest object, my helmet and a piece of fabric and wrapped it around my fully loaded school bag, math textbooks and all. This should give you a general size as to how big the external bag should be.
My bag is made in 3 parts, outside bag, inside bag and water barrier. All three of these layers are sewn together to form one nice solid bag. There is an elastic drawstrap at the top that closes it all up. The extender bag is held on to the school bag using two straps with clips on them.
Step 2: Making water resistant
The fabric/plastic layer didn't look so good by it self so I put this layer into another fabric layer.