Trim off any excess fat around the edges with a sharp straight edged knife.   Slicing against the grain, slice the meat as thin as you can get it, ideally into thin bite sized strips, about 2 or 3 inch lengths (5 to 7 cm).  Alternatively you could buy meat that is already cut into stir fry strips, but I find that this this is usually always tougher that buying and cutting my own.