This is how I have done it in the past.
With a sharp knife, make 1 inch deep cuts into the pork.
With mortar and pestle crush garlic, oregano and black pepper together. Add olive oil, vinegar and salt. Mix well.
Spoon some of the garlic mixture inside the small cuts around the pork, and spread remaining mixture all over the pork.
Place pork in an aluminum turkey pan (skin side up); cover with aluminum foil tightly. Refrigerate at least 8 hours. (I have marinated this for as long as two days.).
Cook at 325 degree Fahrenheit for about 5 to 6 hours.
Remove aluminum foil and bake at 375 degrees Fahrenheit for another hour or until skin is crisp. Closely watch this so you don't burn the skin (chicharrone).
Hope it helps