Here is a good tutorial..just make sure the fabrics, thread & batting you use are 100% cotton!! The poly blends will melt in the microwave and catch on fire. My daughter bought one at bed bath and beyond and it caught on fire the 1st time she used it!! I think it was imported so who knows what the lining was. Also don't wash it in detergent that has fragrance in it. It will retain it in the fabric and transfer to the potato when its cooking, never use any fabric softner either!!
Happy Stitchin' crafty_linda_b
http://tipnut.com/make-your-own-potato-baker-bags/