Place the meat horizontally in front of you on a clean work surface. Cut a one- to two-yard length of butcher's string. Beginning at the leftmost tip of the tenderloin, pass one end of the string underneath the meat one to two inches from the tip, cross the ends of the string on top of the meat, and tie a knot. Trim the shorter length of excess string close to the knot. Hold the remaining, longer string above the meat with your left hand, and loop it around your right hand.