Step 1:
Place Sliding Mechanism over the door, then close the door and the Sliding Mechanism should be able to slide back and forth on the door.
NO drilling required
Step 2:
Hold the thickest Locking Part with the two holes (4 included) to the door frame. Close the door and slide the Sliding Mechanism to see if the edge of the Locking Part catches when you open the door. If it does not catch, you will need to use a 2nd Locking Part by stacking two Locking Parts together.
Step 3:
Once you have figured out the thickness for the Locking Part to work correctly, use a pencil to mark the location for the two screws. Start the hole for the first screw and then remove the screw. Load the Locking Part into place and reinstall the screw to make sure it lines up with the door frame. Once lined up, tighten screw and install second screw.