How do you de-vein king prawns?

It is along the "back" or larger side of the prawn - it may not be black if there's not any poo in the prawn. If the prawns were already peeled & split down the back, then they have been de-veined. If not, then just take a small paring knife & slice down the back of the prawn about 1/2 way thru & then use a fork tine to pull out the vein. After a couple, you'll see just how easy it is.