Loosen and remove the bolts that hold the starter recoil pulley housing to the top of the lawnmower engine. The number and size of bolts depends on the mower model. Three to four bolts measuring 3/8 inch is normal.
Turn the housing upside down to access the pulley. If the cord is still intact, pull it all the way out and cut off the cord near the pulley. Next, pull out the remainder of the cord from the pulley end. If the cord is broken, pull out the remainder from the pulley end. Rotate the pulley until the hole on the pulley is lined up with the hole in the housing, and clamp it in place with a set of vise-grip or locking pliers.
Thread the end of the replacement pull cord through the opening in the housing, starting from the outside, and then through the hole in the pulley. Tie a knot in the end of the cord at the pulley end. Remove the locking pliers, and the pulley will snap back into position, winding the necessary amount of cord around it.
Place the housing back on the mower and bolt it in place. Thread the end of the cord through the guides on the mower handle, then through the hole in the pull cord handle. Tie a knot in the end of the cord at the handle end.