Close the screen door and latch it if possible. Face it from the inside.
Hold the pneumatic door opener up to the top of the door. One end attaches to the wall between the screen door and the main door, while the other attaches to the screen door itself. Pneumatic door closers have brackets on either end that allow them to be mounted directly to the wall.
Mark the spots where the brackets will be attached to the wall and to the door using a pencil pushed through the mounting holes.
Pre-drill the screw holes at the spots you have marked using an electric drill.
Attach the pneumatic door closer to the screen door and to the wall using screws.