Lay your steak flat on a cutting board. Butterfly the steak by cutting in half, stopping just before you cut all of the way through. Then, open the steak up and flatten it out.
Using a meat mallet, pound out the edges of the steak first, working your way towards the middle. You want to pound the steak thinly, but not too thinly. Once one side is tenderized, flip the steak over and repeat on the opposite side. A steak properly tenderized with a meat mallet will start to thin and spread out.
Notes
For the most tender steak, follow these tips:
Let your beef come to room temperature before cooking. This ensures it cooks evenly throughout, which is important for achieving the most tender slices of beef.
Let it rest before cutting. After you cook it, let your steak rest before slicing. If you slice too early, it will lose some of its juiciness.
Slice against the grain, always! The grain refers to the lines running through a cooked cut of beef. Cutting against the grain is so important for tender steak.