In a small bowl, mix together kosher salt, black pepper, garlic powder, onion powder, and any other desired spices or herbs.
Start by patting the pork shoulder dry with paper towel. Then rub the spice mixture all over the pork shoulder, making sure to cover all sides.
Place the pork shoulder in a large resealable plastic bag or wrap tightly with plastic wrap and place in a large baking dish that the pork should will fit into. Place the airtight pork shoulder in the refrigerator for at least 12 hours, and up to 24 hours, to allow the dry brine to penetrate the meat.
Remove the pork shoulder from the refrigerator and rinse it thoroughly under cold running water to remove any excess salt. Pat the pork shoulder dry with paper towels.
Once the pork shoulder has been dry brined, you can season the pork for roasting as you desire. I love to use my Brazilian steak seasoning blend, but you can use salt, pepper, garlic powder, herbs (thyme, rosemary, etc), onion powder, or any other combination you like. Coat the pork with your seasoning and place in a roasting pan...fat side down for a crispy top or fat side up for the flavor of the fat rendering down into the meat. I prefer fat side down. Preheat the oven to 450 F degrees and place the pork shoulder uncovered in the center shelf of the oven for 20 minutes. Turn down the heat to 250 F degrees after 20 minutes and cook for 4-5 hours for a 3 pound roast.(4 pound pork roast: cook for 4 ½ to 5 ½ hours. 5 pound pork roast: cook for 5 ½ to 7 hours.) Test the internal temperature in the middle of the roast to see if it has reached 180 F degrees.
Remove the roast to a platter and let the roast rest for 15-20 minutes. After it has rested, you can slice it on the platter.
Meanwhile, you can make a delicious sauce with all of the browned bits and drippings from the roast. I first pour off the excess fat. There is a lot of fat that renders from the pork shoulder. Then I usually just add ½ cup to 1 cup of boiling hot water and start whisking to scrape up the bottom. It will likely be very salty, so you don't want to add more salt. But, you can certainly use unsalted broth or stock instead of water.