STEP ONE: Find the right kind of bag.
Look for one that has a more rigid outside, like leather or coated nylon, which is what I am using here. Then, look at the innards. You want to find one that has a larger base, with SEAMS going down the middle and across the two ends. Lastly, check to make sure you are comfortable with the strap. You want to make sure you can wear it AND shoot with it on.
STEP TWO: Turn the bag inside out.
STEP SIX: Wrap the fabric around the fabric and sew ends closed.