If your shrimp are shell on, peel them. You can opt to leave the tail on or off. Save the shells to make broth.
To devein the shrimp make a shallow cut along the back and remove the black vein
Melt the butter or heat the olive oil in a skillet big enough to accommodate the the shrimp in one layer. Add the garlic and red pepper flakes and cook for just a few seconds. Turn the heat up. The pan should be hot enough (not smoking) for the shrimp to start cooking on contact.
Use tongs or a wooden slanted spatula to move the shrimp around every few seconds, letting them sear on both sides. Taste one after a minute. They should snap and just be cooked through.
Do not overcook shrimp! Especially if you’re planning on using the shrimp in another preparation once cooked.
Squeeze a bit of lemon juice over the shrimp and toss in the chopped parsley. Eat immediately or save to use in another dish.