I have been using a prototype as often as practical for school and it has performed admirably--when not failing. It took a few weeks to recognize and repair most of the weaknesses and has operated stress free for most of the semester. It's also survived a couple of semi-rigorous bike rides.
This tutorial is based on the bag I've been using, but with important changes that will improve durability. Also, this is the most basic design I've made. Version 2 can be easily customized to suit the needs of the builder.
Here are pictures of the model currently in use (v2.2) and the one made for this instructable. Version 2.2 has taken some abuse and has underwent many, many repairs.
Don't let the 19 steps daunt you--making a messenger bag from scratch can become complex. I've attempted to simplify it, despite the many steps.