Do what M Rose said above, then after you do that apply some Seam Grip on the material where you sewed it on the inside, that should prevent future fraying of the edge of the fabric at your new seam. Let the Seam Grip dry for several days before use. The Seam Grip has a lot of volatiles in it, I always use it outdoors instead of inside.
https://www.rei.com/product/603034/g...ip-seam-sealer