Decide which vertical sliding window you will be using for the air conditioning unit. Choose a window that has an electrical outlet nearby so you can plug in the air conditioner. Be sure there are no pieces of furniture or other objects in front of the window that will be interfering with the airflow.
Inspect the windowsill to be sure it will be a stable base for the air conditioner. If it is cracked or rotten, replace the sill before installing a window air conditioner.
Raise the window all the way up and install the metal support bracket that came with the air conditioner. Check your owner's manual for the exact installation instructions. Use a level to make sure the bracket is straight. Most types of air conditioner mounting brackets have a screw in the middle that can be used to make adjustments and level out the bracket.
Lift the air conditioner up and slide it into the support bracket. Window air conditioners can be quite heavy.
Pull the window down and mark where the window hits the air conditioner. Raise the window back up and attach a strip of weatherstripping foam along your marked line. If there are gaps between the sides of the unit and the window frame, pull out the side extensions. Depending on the type of air conditioner you have, the side extensions may need to be attached to the wall with wood screws.
Apply caulk to the exterior of the home, sealing the edges where the air conditioner meets the window frame. This will keep cool air from leaking out through the gaps while you are running the air conditioner.