If your lawn mower pull cord snaps, you have two choices: Try to fix it, or go for a jungle theme in your yard.
Fortunately, fixing a mower cord is something most do-it-yourselfers can tackle on their own. Here's a step-by-step guide for changing out your pull cord.
1. Remove the housing
Most cords are cotton or braided nylon, and will eventually snap thanks to the combination of hot weather and sharp, repeated tugs needed to get a mower engine running. When they do, your first step is to access the cord's pulley; ideally, you can also remove and save the plastic "T-shaped" handle attached to the old cord.
Every machine is different, but what you're looking for are the bolts which attach the engine's starter housing to the frame. These may be hiding under a gas tank or other plastic parts, and you'll likely need remove five or six bolts to get the housing off. Your best bet here is a ratchet set with an extender, rather than trying to fight with oddly placed bolts.
2. Remove the pulley and old cord
Once you have the housing removed you should see a circular starter pulley. This pulley can be removed, and will contain the remnants of the old cord. Unwind the cord and remove it, which will likely be held in place with a knot.
If you can't pull it out by hand, you may need to cut it. If the pulley is filled with dirt or debris, clean it out, since these will cause wear and tear on the new cord
3. Attach the new cord
Standard, 1/8 inch cotton cord is the cheapest option available and will cost less than $5 at most retail stores. You can also choose nylon cord if you want greater durability. If the entire cord is rotten and you can't even save the handle, consider buying a replacement kit specific to your brand of mower.
These come with the right size and length of cord and a T-handle already attached. Purchase a kit online, or find one at the same store where you originally bought your mower.
To attach the new cord, thread it through the starter pulley hole and then tie a knot at the end. Try to pull the cord back out. Try hard; you want to make sure the knot is tight enough that it won't pop through the pulley hold when you're staring the mower.
Next, wrap the cord around the pulley several times. Don't turn the pulley itself - you want its springs to be relaxed when the cord is sitting idle. When you pull the cord the pulley spins and builds tension. This yanks the cord back into place when you let go.
Wrap the cord around the pulley several times (without spinning the pulley), then attach the handle and give it a test pull. You should be able to fully extend your arm when pulling before you feel significant tension in the cord and it should recoil quickly into the housing.
If you don't use enough cord, you'll feel tension too early; if you use too much, the cord will be slack. Once you're satisfied you have the right length and tension, reattach the engine housing and the cord. Most mowers will have a bracket or small metal loop to hold the cord in place at handle height.
Thread the cord through, and make sure it sits easily, but with enough tension it isn't hanging limp. Start and stop the mower several times to make sure the cord won't break.
For jobs more complex than a mower pull cord repair, consider hiring a lawn mower repair professional.
Major work on the engine, for example, is something you don't want to tackle alone, but with a little effort and a fair amount of patience, you can easily replace your mower's cord.