Garage looks like a car being parked inside a garage, with your non-dominant hand acting as the garage, palm open and facing the ground, at chest height. Then make your dominant hand the "car", with your thumb, index and middle fingers extended out of a fist and then moving back and forth under the non-dominant hand.
Often when you are making the garage sign, you are carrying your baby and have only a single hand free. In these instances, you can just do the part with the dominant hand's "car".
Many children will take an interest in the names of the different rooms in your house. Every time you go into the garage, show your baby the sign for them to make the association.
We introduced the sign for garage as part of our signing deck for going out. We would tell our little ones we were about to head out to the park, so Mama had to take the car out of the garage. We also use it during play sessions with our little boys.