Getting all the moisture out of the rind is one of the keys to success. Pat it with paper towels and if possible leave it uncovered overnight in the fridge.
You want to cut deep into the fat but not into the meat so take your time to gauge how far to cut
The puff of crackling is created when salt reacts with the fat.
Make sure you rub salt and oil liberally into the score marks.
When it comes to choosing a cut of pork, start with a pork belly, being a more marbled cut, the meat is less likely to dry out with overcooking. A pork shoulder is also going to be more forgiving of cook time while you're learning. Whatever cut you choose, try to get the fat as level as possible in the oven as this will help create an even crackle across the whole piece of meat.
If your recipe includes a marinade, like this Sticky Pork Belly, then make sure you keep it to the meat only, the fat needs to stay dry to get that crackly magic.