Thanks, guys! 
I just used good ol’ fashioned school glue, applied thinly on the outside of the folders a small-ish area at a time. (I used an old loyalty card to spread the glue on the folder.) Then I smoothed the fabric over the glue and then laid them out to dry. You do have to be careful to maintain that fold in the center, of course. After they dried, I stitched around the edges, then trimmed the fabric. After I had sewn a couple, I realized that it would be wise to make sure that the fabric was glued all the way to the edges of the folder to help prevent the fabric from shifting, so a little more drying time was required.
Once everything was stitched and trimmed, I ironed all of the folders. (The pic was taken before ironing and I neglected to get an “after” pic.) I used white glue as a fray stop around the cut edges of the fabric, simply applied a small amount with my finger tips.
I like the fusible webbing idea @steiconi! I’d love to see someone try that option.