Make sure the lock still lines up, then place the ground section into the hole, and pour your ready-mixed concrete around it. Make sure it's held straight until the concrete sets. Now, you can fully tighten the door lock bolts, and you're all set.
Depending where your garage is, you might want to paint the lock to match the door, so it's less obvious that the security's been increased. Sometimes, a garage door with big locks will attract even more attention – crims will think there's something worth nicking inside.