Once you spot one, simply run it over to install that mod on your current car.
Modding a vehicle in Fortnite is so simple you may even do it by accident the first time around. While you're driving around the map, keep your eyes peeled for the vehicle mod crates lying around. These will have some text above them telling you which mod they are. Once you spot one, simply run it over to install that mod on your current car. You can also pick them up on foot and throw them at a vehicle to equip them that way.