You only need to drain about 1 gallon out. Here's an easy and less messy way to do it. Fill a bucket with water and completely submerge a plastic or rubber tube in the bucket and allow the tube to fill with water. While the tube is underwater, place your fingers over each end of the tube. Place one end in the radiator and hold the other end of the hose until you have placed it in an empty plastic jug on the ground. This starts a siphon and you won't need to drain from underneath the car.