This video shows how to prepare a flank steak for searing. First, the tough skin on the outside of the steak needs to be removed to help improve tenderness. When slicing flank steak it is important to notice which way the muscle grain is running and slice across the grain. Slicing with the grain will give you a very tough piece of meat.
Take a sharp knife and score the steak in squares on both sides. This allows the salt or marinade to penetrate, and also helps with tenderizing. At this point the steak is ready to be marinated, overnight if possible.