The pull cord on a weed trimmer cranks the engine and flywheel, which fires off the spark plug, igniting the chamber. Without this pull cord you wouldn't be able to start the engine. The pull cord is also wrapped around a pulley, which is tensioned by a recoil spring, and if either of these parts are broken, you will also have difficulty starting the trimmer. When the pull cord is working properly it should fully retract and be neither too difficult to pull nor too easy.
Use the screwdriver or Allen wrench (depending on model) to separate the engine from the shaft.
Take off the engine and unscrew the ignition module on the belly of the engine. Use the screwdriver or Allen wrench to remove the starter cover and open the engine. Remove the starter cover.
Turn the starter cover over and release the center-locking tab. This tab connects the rope to the pulley and must be released before you can remove the cord. Pop it open with the screwdriver.
Use the screwdriver to pry open the plastic starter handle. Remove the metal insert and pull cord from the handle. Cut off the old cord, pull it off the pulley and throw it away.
Stick the new cord in through the hole in the center of the pulley. Pull it all the way through and close the tab on the cord. Pull on the cord to make sure it will withstand hard yanks.
Push the cord through the hole on the starter cover, the center hole on the metal insert and the starter handle. Tie a loose, overhand knot around the metal insert. Don't close the knot.
Place the metal insert and cord into the starter handle. Close the starter handle and pull down on the string to close the knot around the metal insert.
Wind the pull cord around the pulley. While holding the pulley and starter cover in one hand, pull out on the cord for two full pulley rotations to set the recoil spring.
Screw the starter cover back in place. Reattach the ignition module and engine to the trimmer shaft.