1--cut your fabric into rectangles--I do them about 4x8inches.
2--fold the rectangle in half with right-sides together, and sew two sides (I usually serge them because it's fast and easy, but a regular straight seam is great, there's no need to finish the edges.)
3--turn the bag right-side-out
4--fill it about halfway with beans, lentils, split peas, rice, or wheat. (I prefer to use lentils or split peas because they are cheap and make for softer beanbags than the bigger beans to. Softness is key since I have boys throwing the beanbags at everything including me!)
5--fold the open edges under, and pin in place.
6--sew it shut
(This is why you don't want to fill it more than 1/2-2/3 full, otherwise you will not be able to keep the beans in while you sew!)