Cut pork shoulder roughly into 2 inch pieces.  Add to a non metallic baking dish. The pork will marinate and cook in this dish.

Add orange juice, juice of the limes and achiote paste.  Stir thoroughly until well mixed.

Pour marinade over pork and coat well.  Cover and refrigerate for upto 24 hours, the longer the better.

Preheat oven to 360F.  Remove pork from refrigerator, remove cover and recover with foil.  Seal tightly.  I like to repeat this step a second time, sealing the pork with a double layer of foil.  The pork cooks slowly at a low heat, so we want to lock in all the moisture and heat.

Cook for 45 minutes at 360F and then reduce heat to 325F.  Cook for a further two hours and 15 minutes for a total of three hours cooking time.  Remove from oven and test pork.  Pork should shred easily with pressure from a fork. Cook for another hour for more tender pork.

I like to transfer the cooked pork to separate bowl to shred with a fork - and also to remove some of the excess fat and cooking mixture.  Serve with pico de gallo, diced white onion and cilantro for perfect tacos!