I have spent the day sewing up some homemade microwaveable heating pads for some Christmas gifts. Very easy and quick to make! You have to use 100% cotton fabric and I filled them up with rice.
Take two pieces of fabric that are approx. 12 inches by 15inches and sew them with the right sides facing each other on 3 sides. Turn them right side out and then measure every 2 inches. I used a fabric pencil to make the lines. Sew them leaving about an inch at the end. Fill each tunnel with rice. Then sew the end shut neatly.
That is it!
Place them on a microwave safe plate and into the microwave for about 1 minute 30 seconds. Use the warm pad on your sore back or whatever ache you have.