Disclaimer: I am not an expert on making or using these bags. You are at your own risk if you choose to make them. I strongly suggest you to go to the link that I provided for more detailed information.
It is important to use 100% cotton batting. (I recommend using The Warm Companies batting to be safe.) The bag will catch on fire unless you use 100% cotton!
To be safe use 100% cotton thread. I am not sure if polyester is OK.
Do not leave microwave unattended when in use.
I would not use the bag if your microwave is very hot.
If your microwave has a turntable; the bag must rotate freely or it will burn.
If you don't have a turntable, turn the bag over at half time (suggested cooking time) to prevent scorching.
Wash and dry the bag before use.
Do not microwave the bag unless food is inside it.
Make sure the finished bag will clear at least 1 inch all the way around in the microwave.
Credit: The Warm Company I could not get the link to work so Google The Warm Company. Their site offers a lot of reviews and information along with the free pattern.