Cutting and Sewing Denim

Hey all. I’m a total leg amputee with no prosthetic so I either have to roll/fold the pant leg up or I need to sew shut the side after cutting off the pant leg. I’ve been cutting/sewing and learning how to do that but after washing them, the denim is fraying past the stitch.

What do I do to stop this from happening? I have a Singer Heavy Duty machine. Or should I be doing this hand?