"Find where your air filter is located. While the central air conditioning unit, or condenser, is located outside the home and connected to the home through an electrical and airflow system of wires and tubes, the filter is usually located in the home Pull or open the filter compartment and get it out of the air conditioner. Some filters are covered by a metal strip that must be removed to gain access to the filter. Remove all the screws holding the compartment in place, and have them in a safe place so you can find after replacing the filter. Measure the filter’s outer dimensions, and look for any identifying numbers or letters on the casing. Take measurements or the old filter with you if you’re not sure what size filter you need. Slide the air filter into your air conditioning unit the same way the old one came out. Make sure the filter is placed into the compartment with the correct side facing out. Follow the directions on the package. The sides are usually marked as air-in and air-out."