Assuming, you have a whole prawn, with head attached. Mine comes without the head. The quickly way to behead a prawn is to pinch the base of the head and tear it off, then trim off the guts. You can throw away the head. My parents used to boil it and suck out the inside. They loved it.
Colossal prawns I bought from Whole Foods come without the head. My goal here is to take off all the legs, scales, guts, but leave the tail.
2. Then, position the prawn with it's back towards the top. You should see a dark "vein" just below the skin through the middle of the prawn from tail to head.
3. With a small pairing knife, cut into the prawn about ⅓ way deep (the knife will meet some resistance and that's where you stop). Then, slice shrimp from ⅓ from the tail all the way towards the head (or the lack there of). The pros slice the entire length of the shrimp to clean the entire thing, but it is not essential.
In in the prawn's back, is this dark blue/black/green "vein". Using your knife, tease out the entire length of this vein and pull it out.
Done. It's rather easy. It might be a little gross and intimidating the first time, but it gets easier.
Cooking prawns is even easier. Mix the prawns with salt, pepper, and whatever store bought savory sauce or spices you want. Cook it in a pan over medium heat with some olive oil. It is done when it turns pink and becomes a little stiff.
TIP: Some fish monger measures the size of prawns using strange numbers such as 16/20 or 18/24. These numbers indicates how many prawns you get in a pound. So 16/20 means you will get 16 to 20 prawns in a pound.