Here is a link for a bag that I have made at least 5 times. The directions are very clear and easy to follow. You can customize what you like...the base size of the bag, strap length (adjustable or not), pockets, etc. I made mine with outdoor fabrics on the outside with cotton lining. It is pretty darned easy and I love the "hands free" style of the messenger bag.
http://www.craftsy.com/pattern/sewin...22536~e4c7ne7g