There is no wrong way of doing things. Most of us did the way our mothers taught us, while some learn from cooking shows and the likes. This is how I did:
A pair of scissors
We are using the scissors first. Hold the scissors in the un-gloved hand, and use toe gloved hand to handle the prawns. Trim off the sharp head, or as I call it, the punk hair of the prawn.
Cut off the whiskers and the sharp hornlike thingy.
Trim off the sharp part of the tail as well. Don't cut off too much though, it violates the aesthetic principle.
Cut off the legs too. Repeat steps for all the prawns. When that is done, put down the scissors.
Grab the knife and skit the back of the prawn. Remove the veins and the dirt. After that, the prawn is ready for cooking.