I use a deep well socket that is slightly smaller in diameter than the outside of the seal. I turn it upside down so the hex end is facing up. After lubing the seal, I tap the inverted socket with a hammer until the seal is seated. I also watch the deep well socket as I drive it in to make sure that it is going in straight.
Like the others have said, it just needs to be in far enough for the retaining clip to be installed.