Unplug the air conditioner. Remove the filter cover. Cover styles vary by make and model, but they are positioned across the front of the unit and usually feature a handle or lip on one side. Remove the filter.
Vacuum the filter. For thorough cleaning, fill a kitchen sink with warm water and a splash of mild dish detergent. Soak the filter for 15 minutes to loosen dirt and dust. Run the filter under a faucet or sprayer until clean. Shake the filter dry and place it back inside the unit. Repeat monthly or as necessary.
Straighten the fins on the back side of the unit with an air conditioner fin comb according to package instructions to open the air passages. Repeat as necessary.