For this particular one (that went out in my giveaway) I made an inner pad with a washable slipcover. This slipcover is actually a women's knee high sock I cut the foot off of.
First cut the foot off your sock. Turn it inside out and hem the open, cut end with your machine and a ball point needle. Turn right side out again and put aside.
Now take a piece of fabric cut to 12.5" x 8". Turn right sides together and sew along one short side and down the long side. Turn right side out. Fill with dry long grain rice. I used a tall glass to put the fabric tube in and use a small measuring cup to fill it. Leave about 2-3 inches from the top. Fold in open end and sew with 2 straight stitches. Now pull sock over the rice pad and use or give as a gift.
Idea: Make some with gender neutral socks/fabric and give as a His and Hers pads.
To heat: 2 minutes in the microwave will make it nice and hot. You can also leave in freezer for cold therapy.
*Edited to add: if your sock has silver, shiny threads, be sure to remove sock portion before microwaving! :)
Now you don't have to make long skinny or sock ones. This rectangle one was made for my oldest son years ago (he gets tummy aches with his fevers). It is made with fleece (though I don't highly recommend due to it seems to 'sweat' when hot, but it works fine anyway). To make a rectangular one, just sew 2 rectangles right sides facing as making a pillow and leave a 'hole' on one side for filling. Then fill and sew up the open seam. This one has seen many upset tummies from all of us in our house. I really need to make a few more. :)