Measure your garage. Determine how many square feet cover the entire room. Once you have measured the space you want cooled, determine the size of air conditioner you'll need. A good rule of thumb to follow is that a 12,000 BTU unit will cool a 500-square foot room or garage.
Determine what window you want your air conditioner to go in, in your garage. Measure the window opening. Be sure to take you measurements to the store with you when you purchase the air conditioner.
Go to a local store that sells air conditioners and purchase the one that will fit your window opening and best cool your garage. If you are having trouble finding an air conditioner, look at either Home Depot or Walmart-- both of these retail stores sell a wide variety of brands and sizes.
Take all instructions and pieces out of the box, including the air conditioner itself. Place everything on a flat surface so you will be able to reach it easily while working. Find the mounting brackets that should have come with the air conditioner you purchased. By following the manufacturer's directions, install the mounting brackets.
Put the air conditioning unit into the open window and directly attach it to the supplied mounting brackets. Again you will need to follow the manufacturer's directions as each air conditioning unit is different. Pull the window pane down until it closes on the top part of the air conditioner.
Pull the extensions out from the sides of the air conditioning unit then attach them securely to the window frame. Most extensions will snap into place with snaps that are directly on the unit. Be sure to not allow any outside air in as this will make it difficult for your unit to work properly and most effectively.
Using a piece of foam, create an air tight seal between the window pane and the top of the unit.
Use caulk in order to keep the warmer air on the outside of your garage. Place a thin bead of caulk on the outer side of the air conditioning unit.
Plug the air conditioning unit in and the cool air will begin to fill your garage.