Mix the flour with water and knead it for about 20-25 minutes or until the dough gets soft. Separate the dough into two equal portions and roll them into cylinders (about 1 inch in diameter). Cover them with wet towel and set aside. To prepare the skin, cut the dough into 1/4 inches length and use a rolling pin to flatten it until it becomes a round skin about 3 inch in diameter.
Combine the ground pork, shrimp, chopped napa cabbage and seasonings together. Set aside.
To make potstickers, place a small spoonful of the filling in the center of the skin. Dab a little water with your finger and circle around the edge of the skin, and then fold and pleat the potsticker accordingly. Repeat the same for the rest. (Please refer to this video or this guide for folding/pleating potstickers.).
To pan fry the potstickers, coat a frying pan with a little cooking oil and turn to medium heat. Place the dumplings on the frying pan and then turn the heat to high. Pan fry the potstickers until the bottoms turn golden brown and crisp. Add the water and cover the frying pan with its lid immediately. Cook until water has evaporated and turn the heat to low. Cook the potstickers for another 2 minutes or so, dish out and serve hot with Chinese black vinegar.