A real messenger bag has a stabilizer strap. Without the stabilizer strap, it really won't work. Unless Tom Binh is reinventing the wheel (and I wouldn't put it past him), the arrangement is a very simple one:
1. Shoulder strap over the head. Let's say the bag will hang on your right side (well, actually it's behind you, but you get the idea), so the strap crosses diagonally from your left shoulder to your right hip.
2. Stabilizer strap has one attachment point at the bag's lower right corner (if you were looking at it facing the back side of the bag with the bag upright). The other end of the strap has a connector that connects to a gizmo located on the shoulder strap. Usually this gizmo has some ability to float around, but it ends up more or less in the middle of your chest.
3. Stabilizer strap goes from the lower corner of the bag, which in this example rides by your left hip (plus or minus, some wear it way high, doesn't matter) around your left side, and clips onto the shoulder strap more or less at mid-chest.
Or
look at a picture.