I didn't have anything to re-purpose, so I used some place mats I purchased from Target on clearance. The first thing you need to do is figure out the size you want to create your sandwich bags. I like Brownberry bread, which tends to be winder than say Wonder Bread. So, I cut my fabric a 8" width and 15" length.
I sewed a 1/2" hem on the top and bottom of the length of the fabric. Then I folded a 1/2" hem on the edges of the long sides and folded the bottom up to create a pocket. When you create your fold, make sure that the top will fold down over the pocket by 1-1.5". Stitch the hem with the pocket folded in place and top laid open flat. Attach velcro to the center of the top and pocket.
I do have to admit that my stitching job is not the prettiest on this. Due to the plastic nature of the fabric it did not pull under the foot very easily and would stick, so there are spots where the stitch that was doubled or tripled. And because the plastic fabric would leave holes where the stitches were placed if I removed them, I just left the stitches as is.