I've been searching for an army knapsack/haversack for a looong time, and the ones I find in surplus shops cost absurd amts of money. So I figured I'd buy canvas and make one.
Problem: I lost the fabric :P.
One say when I was visiting the thrift store, I found an old army laundry bag, olive drab with a few holes. It was only 2 bucks so even if I never used it, I figured it wouldn't be much of a loss. Problem 2: Too damn big to carry anywhere. So I figured I'd turn it into a usable backpack.
Step 1: Measuring.
After getting all your material and supplies mentioned in the intro, take your measuring stick/tape measure and count:
-7 inches from the top of the bag and;
-12 inches from the top of the bag
Do this all around the entire bag.