If you use it enough, your lawn mower's pull rope is inevitably going to break. Every time you yank on it to start your lawn mower, you put incredible and friction on the rope. Fortunately, it's quite easy to learn to repair the pull rope on a lawn mower--and the skill is one you'll likely use often.
Find your mower's starter pulley. On almost all lawn mower models, it is located on the right side of the engine. And it can always be found by simply following the string. Have your wrench ready. To access the starter pulley, you will likely have to remove the lawn mower's cover by removing a series of bolts. Then, remove the bolts that secure the starter pulley's housing case, pull the case off by hand and remove the starter pulley by removing the bolt that affixes it to the lawn mower. The configuration on your mower may differ slightly. Consult your owner's manual if you are unsure how to proceed.
Remove the broken pull rope from the starter pulley. But first, take not of how many times the pull rope is wrapped around the starter pulley. You will want to duplicate this later.
Prepare the new pull rope. Cut a length from the spool that is just as long as the one you removed. To make threading the ends of the pull rope easier, wrap them with a layer or two of tape.
Look for the small hole in your starter pulley where the old pull rope was affixed. Thread the new rope through this hole from the outside of the pulley in. Then tie a double not at the end of the pull rope to secure it.
Re-attach the starter pulley to the lawn mower.
Wrap the pull rope around the starter pulley the same number of times as the old rope.
Thread the pull rope through the starter pulley's housing case. Then re-attach the case.
Thread the starter pulley through the hole in the lawn mower cover. Then re-attach the lawn mower cover.
Thread the pull rope through any attachments along the handle of the lawn mower.
Attach the end of the pull rope to the handle. This may be as simple as threading the pull rope through the handle then securing it with a knot on the other side of the handle. If your lawn mower handle's configuration is different, duplicate the method by which the old pull rope was affixed.