An engine's sparkplug is an essential part of the ignition system. At the end of the plug is a small gap between the plug and the electrode. Spark jumps across this gap and provides the engine's spark, but over time the metal can get worn, causing the gap to change. This can result in a poor-running, poor-performing engine. If the plugs are reusable you can clean and regap them, or you can install new plugs that will need to be gapped as well.
Remove the sparkplug wires from the car. This will vary from car to car. Consult your workshop manual.
Remove the sparkplugs with a sparkplug socket. A sparkplug socket is similar to a normal socket, but has a rubber insert inside that grips the plug so it doesn't fall out of the socket as you remove it from the motor.
Slide the sparkplug gapper between the tip of the sparkplug and the metal electrode until the proper gap is set. The edge of the plug gap tool gets gradually thicker as it goes around the tool. Some gap tools use wires of different thicknesses to set the gap.
If the gap is too large, press or gently tap the end of the plug against something to reduce it, then check and gap the plug again.
Repeat the gapping process for all of the plugs. Reinstall the sparkplugs and sparkplug wires.
