I have made many.. . . but you don't cook them buy just HITTING the Potato button on your Microwave.
You take a med size potatoe and if you use more than one that side, then you cook them for about (max) 3 mins each. If I have 4 potatoes, in about 1/2 way thru the cooking, I turn the bag over, this is just me. They come out great. I wash them and put a damp paper towel around them before placing them in the bag.
I have made many of them, only never would I do what you did. I have a nephew that put a potatoe in there for 10 mins and it catch on fire, all I can say is never do it that way. In this case less is more.
I used 100% cotton outer fabric, and cotton batting, and mulsin for the inside.
Out of one yard of Main fabric you can cut 6. 12" by 21 long, same with the batting and muslin. If I have the fabric with the picture of potato's on them, then I do an embroidery design, that isn't cotton thread, but I have never catch them on fire or even smoked them.
For something like that don't forget no to microwaves are the same power and it is done of a average of what it thinks it would cook it.
It is just like the popcorn setting, no to microwaves cook that the same.