I use a mixture of 50/50 rice and feed corn. I've made 12" x 12" or 12" x 24" (use what you have that is close) I sew three sides like a pillow case, then sew channels up the long way.
Fill each channel 1/3 then fold over top sew zigzag to close
IF 12 x 24 then mark halfway point on the long, fill 1/3 to that mark, zigzag the half mark, fill 1/3 of the second half and close as above.
I like the 12 x 24 as I can put it under and over my feet at the same time.