Can Anyone Help Me London Broil a Flank Steak?

I am trying to surprise my wife, but have never done this before. Currently I have a 2+lb flank steak from our local butcher, it looks like it has already been rubbed so I am not sure if I still need to marinate it.

Also I tried looking up how to actually do a London Broil online and all I seem to find are vague directions, nothing about how long to cook the meat.