The craft stores have different types of handles you can buy for your new bag. You can even go to your nearest 2nd hand store and purchace and old belt to use as your strap. My straps are made from matching duct tape.
Each piece started out as a 2" wide strip by anywhere from 3" to 5" long. I folded the first piece and just kept adding the short pieces as I went along. I then took my somewhat uneven strip and layed it along a single strip down the center, folded the egdes over each side. I then flipped the strip over and did the same thing so as to cover any raw or sticky undersides. I did both strips this way.
How long the straps are is entirely up to you.