Here are the simple, step-by-step instructions:
First, mix the minced pork meat, pork fat, crushed garlic, ground bay leaf, brown sugar, soy sauce, apple cider vinegar, salt, sweet paprika, black pepper, and red pepper flakes (if using) in a large bowl. Let this mixture rest for 1 hour to let the flavors combine.
If using casings, stuff the meat mixture into the sausage casings. Twist and tie the casings with kitchen string every 4 inches to form individual sausages. If making skinless longganisa, add 3 tablespoons of cornstarch to the meat mixture, then shape small portions into logs about 3 inches long.
Place the sausages in a covered container. If using casings, refrigerate for 2-5 days to cure. For skinless ones, wrap each piece in wax paper before placing in the container, then refrigerate for 3-5 days.
When ready to cook, place your desired amount of sausages in a skillet. Add half a cup of water (it should come up about half an inch in the pan). Turn the heat to medium and bring the water to a boil, then let it simmer until all the water evaporates, about 10 minutes.
Once the water is gone, prick each sausage a few times with a fork. Let them fry in their own rendered fat for 3-5 minutes, until they turn a reddish-brown color and are nicely caramelized. If needed, add a small amount of oil.
Serve your hot longganisa with garlic rice, eggs cooked your way, and sliced tomatoes. Have some spicy vinegar on the side for dipping.