Courtesy of Gastronomy Domine. I (John Simeon) did pretty much this for Christmas lunch in 2012 and it worked perfectly. I can't remember for the life of me where the pork came from or how much there was, but I did do both the hair-dryer and and goose-fat. Felicity Cloake is also worth reading.
The day before you eat, the skin of your pork must be dried thoroughly with paper kitchetowels, and scored well. Even if your butcher has already scored it, you will probably benefit from making sure the scoring is fine and regular, so you will want to add your own cuts to the skin. Use a craft knife on the cold skin of the meat (this is easiest when the skin and fat are cold and firm), scoring it in lines about half a centimetre apart. When the joint cooks, the fat will melt and bubble through those lines, crisping the skin it touches. Rub salt into the skin, as if the pork were somebody you are particularly fond of who is demanding a lovely exfoliating massage.
Now prepare to look slightly unbalanced in front of any visitors, and take a hairdryer to the skin of the meat until it’s absolutely bone dry. Wrap your joint in a teatowel and refrigerate it overnight. (The atmosphere in your fridge is extremely dry, and this will help any more moisture to evaporate.)
On the day you cook it, rub some more salt into the skin, making sure it gets through the cracks where you scored it and into the fat. Put a bed of onions at the bottom of a metal roasting dish and rest the pork on top of it. Heat up a large knob of good pork dripping or goose fat (use goose fat in preference to one of those white blocks of lard) over a high flame in a small saucepan and pour the searing hot fat over the skin, then put the roasting tin in the oven at a very hot 220°C. After quarter of an hour, lower the heat to 180°C and cook the joint for two hours, basting every 20 minutes. Finally, turn the heat back up again for a final quarter of an hour – this should cause your minutely prepared skin to puff up and crackle deliciously. (Keep an eye on it and leave it in for a few minutes longer if necessary.)