Make a tube then stuff it with poly fill or batting up to the elbow point. Then sew two lines across the tube about 1/2 inch apart to make the hinge. Continue stuffing up to the shoulder then sew across again to hold the stuffing in. Make sure you pack the stuffing in really tight so each section is nice a rigid. If you think the spot where the "elbow" is looks odd under a shirt then you can glue a small piece of a foam ball that is cut in half to the bottom of the upper half of the arm. This helps add some shape at the elbow and has the added effect of keeping the elbow from bending backwards in the wrong direction. If the arm is going to be bear without any shirt then make your tube out of something like cotton then cover that in your fleece.
