The Super Easy Method:
  1. Fill a cotton sock with rice.
  2. Tie the end shut. 
  3. Microwave on high for 1-2 minutes or to your heat preference.
  4. Enjoy!

 
Fancier Method adapted from Main Warmers: 
Exact measurements are not vital so you can use those old towels and pillowcases without an excess of trimming!

Materials:
 
1 - 27" x 12" piece of 100% cotton for cover material (soft flannel or terry cloth).  Non-100% cotton materials are more likely to burn in the microwave.  Make sure your material contains NO metal!

1 - 27" x 12" piece of muslin or other cotton material for the lining.

3 cups rice or other grain or beans - just not popcorn!

needle & thread or sewing machine.
 
 
Cover: 
  • At one end of the rectangle, turn the 12" edge under and stitch.  This will eventually be the opening for inserting the lining filled with corn or rice.  
  • Fold the long way with right sides together and stitch the other 2 sides leaving 3/4" seam allowance.  
  • Clip extra fabric from the corners.  
  • Turn right side out.  
Lining:
  • At one end of the rectangle, turn the 12" edge under and stitch.  This leaves a nicer finished edge when the last baffle is filled and sewn shut.
  • Fold the long way with right sides together and stitch the other 2 sides leaving 3/4" seam allowance.  
  • Clip extra fabric from the corners.  
  • Turn right side out.  
  • Add ~2/3 cups rice and stich across the way to create the first baffle.  
  • Repeat previous step 2 or 3 more times spaced evenly to create 3 or 4 baffles.   This will keep the filling from falling to the ends of the flannel bag


Finish:

  • Insert lining with rice inside into the flannel cover.  
  • Make sure to remove all metal pins and needles before placing in microwave for 1-2 minutes.  Adjust heating time to your needs.  
  • You can also store it in the freezer and use it as an ice pack.