Open the window where you want to place the air conditioner.
Place the bracket on the windowsill according to the installation instructions that came with the air conditioner. Most air conditioners come with a single bracket that you must center on the windowsill. Use a measuring tape to measure the center of the sill and mark the placement of the bracket.
Place a level across the bracket and the windowsill to level it. Mark the screw markings on the windowsill through the predrilled holes in the bracket. Some brackets also screw onto the outside of the house. If this is the case with your bracket, mark the screw markings on the exterior wall as well.
Drill holes in the windowsill and wall where you made the screw markings. Make the holes slightly smaller in diameter than the screws.
Screw the bracket in place, using wood screws to attach it to wood or vinyl and sheet metal screws to attach it to metal.