It’s generally preferable to de-vein prawns before you cook them.
I don’t like to fight with my food once it’s on my plate. I’ll occasionally play with my food but not have to start getting picky trying to find remove a yucky black intestinal cord filled with sea bed waste.
So, before you cook them, peel off the shell right through the meaty part of the tail keeping the end of the tail and head intact. Place on its side, run a knife down the back and remove the vein, just like Curtis Stone explains in the video.
Simple!