There is actually quite an art to it, though it involves just three fundamentals: salt, high temperature, and dry skin.
Both fat and rind need to be present on the surface of the joint of pork.
Wash roast if you wish, but whether you do or not, wipe all the skin dry with paper towel, then let stand for half an hour to warm up and dry.
Score the rind evenly, or punch a lot of small holes in it (some people use as a tool a board with nails stuck through it to make the holes.) This scoring lets the excess fat render off.
Rub with oil, and wipe any excess off.
Sprinkle with salt evenly, and rub the salt into the skin and the scores.
Cook the pork roast until the internal temperature reaches around 120 F (49 C.) At this point, you move into the cracking phase. Remove the roast from oven, cover it, and put it back in the oven, cranking the heat up to 500 F (260 C.)
What causes the crackling to form is hot fat pushing its way up and running over the rind.
If your crackling isn’t crackling, slice off the layer of fat and rind, put it on a baking sheet, pop it back in the oven on its own and crank the heat — another 10 to 15 minutes should do it. Cover the pork roast meanwhile to let it rest out of the oven.
Don’t baste the roast while it’s cooking. With the fat on top, you won’t need to — it will baste itself, Plus, the basting may make the crackling tough and chewy, rather than crispy.
The purpose of the salt is both to make it tasty, and to absorb moisture so that it can get crispy.
Literature & Lore
“There is much, much pleasure to be had in honing a dish to perfection… In a piece of roast pork it is the contrast between the sweet, rich meat, succulent fat and crisp, salty crackling.” — Slater, Nigel. Perfect summer pudding. Manchester, England: The Guardian. 5 August 2001.