Question: What is the best way to seal a garage door?
Answer: We assume you are referring to a sectional garage door rather than the tilt type.
Generally speaking, a high-quality sectional garage door that has been properly installed is reasonably weatherproof. But even the best doors can begin to leak after long use.
A simple fix is to apply a 1-inch-wide strip of rubber to the inside surface of the door at each horizontal section joint, from one side of the door to the other.
Apply the strip to the upper section centered between it and the section below. A special rubber bottom can be purchased to fit the base of just about any garage door.