Disconnect the negative terminal on the battery.
Locate the battery either in your engine bay or in the trunk. It will look like a black plastic box with 2 metal terminals sticking out of the top. Find the negative terminal, which will be marked with a (-) minus sign. Loosen the bolt holding the cable on the terminal and then slide it off.

Use 220-grit sandpaper on the electrode.
At the end of the spark plug (on the side that goes into the engine) you’ll find a small piece of metal extending out of the plug. That’s called the electrode. If it is black or discolored, slide the sandpaper beneath the bent over portion of the electrode between it and the plug itself and run it back and forth until you see clean metal on either side.