use sturdy fabric (denium, heavy flannel, even terry cloth) and make a bag about 2 1/2 inches wide and as long as you want your pad to be.
Sew up all 3 sides and most of the fourth. then fill with either flax seed or brown rice. Fill it 3/4 full.
Stitch up the remaining and it is ready for use.
Instructions for bag:
Microwave one mintes at a time until it has reached the desired warmth. Shake the bag after each mintues it has been in. be careful not to overcook.