Look around your garage for evidence of a skunk residing. The awful odour might be a good indication that you have a skunk living in the garage.
Leave garage door open:
Leave your garage door open at night and there is a good chance that the skunk leaves. You can always put powder around the entry of the garage to pick up any footprints the skunk might make. If you see footprints leaving the garage then immediately close the door so that you know the skunk is outside.
If you have a dog then keep him inside the garage at night or have him sit in front of the garage door. His barking will definitely scare off any skunk from entering your garage. Be sure to keep your dog on a leash as you do not want him to hurt the skunk.
Soak some pieces of cloth in ammonia and leave them around the inside of the garage. This irritates skunks and will hopefully encourage him to leave your garage. Be sure to place these ammonia soaked cloths all around to cover the entire area of your garage.
Put some pet food outside the garage and leave the door open. Hopefully this will entice the skunk to leave the garage as he goes outside to eat the food. Once the skunk is outside be sure to immediately close the garage door.
Go to a hardware store and buy safe harmless traps that you can set up in your garage. These traps catch the animal without hurting him. Once you catch a skunk always remember to put a cloth over the trap before transporting him away from your area. Remember to release the skunk in a wooded area.
If all else fails you can always call a professional who can come out to your house and remove the skunk from your garage. Remember that this can be costly.