Attach the sprinkler to the end of the garden hose.
Stand at the front of your car with the sprinkler in hand. Set the sprinkler down and push it underneath the car. Feed in the hose to push the sprinkler all the way to the back.
Turn on the water.
Pull the sprinkler closer to the front of the car by about one foot after about five minutes of spraying.
Continue advancing the sprinkler towards the front of the car at a rate of about one foot every five minutes. Keep this up until the entire car undercarriage has been sprayed thoroughly.