There are four simple steps to getting crisp crackling: Score the rind with a sharp knife, rub it with salt and oil, roast for 15 minutes and then do not baste the joint for the rest of the cooking time.
Score the rind: width wise with a very sharp knife or a scalpel, keeping the lines parallel and close together. First work from the middle toward one edge, then turn the meat around and work from the middle to other end. This is easier than scoring in a long line. Always tilt the knife blade away from your other hand and make sure the knife cannot slip toward your body.