What you need:
Old jeans. The softer the better, but not too threadbare or you may lose some rice.
A sewing machine. Not strictly necessary but makes life easier.
Tea. Makes everything easier.
These jeans were the husband's. They have a hole in an unfortunate location and so now belong to me.
I trimmed the inner seam to make the heat pack a little more narrow than the full width of the leg. It ended up being about 7" wide and almost 2 feet long.
Next I sewed the end shut and compartments for the rice so it wouldn't all bunch at one end. I used yellow thread in a zig zag stitch because that's the way I roll.