So, I have this messenger bag (Mark2) that I need to use as a scout bag on a camping trip that I'm going on soon. Since messenger bags have a tendency to get in the way when you don't want them to, I need to turn this thing into a rucksack but I don't want to loose the messenger bag quality....so...enter the Dr. Franken-bag (heretofore to be referred to as Mark4 (Mark3 was a complete bust and didn't make an Instructable about it, which is why you're asking yourself, "Wait '4', where's 3? What did he do for Mark 3?").
I got myself some rope (about 6 feet the thicker the more comfortable; this stuff is half-inch (about 13 mm for you forward thinking folks who decided base 10 make adding easy)). I measured it by wrapping it around my back and holding it out in front of my in both hands so it made a bit of a horseshoe shape around me.
I then ran one end through the D-ring on one side of the bag and tied a double overhand knot to secure it as a sliding knot. This will allow me to shorten or lengthen the strap as needed. Sort of like, it's technical, but it uses Transformers-like technology to accomplish that part. Luckily, the patent ran out on that like 500 years ago, now all ropes can utilize sliding knots license free. WOOT PUBLIC DOMAN!!!
Lather, rinse, repeat for the other side.