I've done this one many times, but if the skin is particularly tough to cut, in its raw state, I put the larger pieceof skin into the oven, then take it out after about 5 minutes, by which stage it's cuts like butter. If you do the latter, please be careful because by the nature of the hot oven, it will be very hot when you taken it out to cut it.
Once cooled, render down either in a mortar and pestle or blender/grinder/food processor.