Window sash locks can be important on your windows for a few reasons. The main reason is for security so the window can't be opened from the outside by would-be intruders. They also can't be opened from the inside, when in the locked position, which can be important if you have small children that could fall out the window and be hurt or killed.
Prepare the Window
The sash lock is installed on the center portion of the window, where the upper and lower sections meet. Determine the center of the upper surface of the lower sash (the part that slides). Mark it with a pencil.
Most sash locks come with a template, however, if yours doesn't have one, use the lock and center it over this pencil mark. Mark where the screw holes should be. Align the part that the lock slides into on the stationary part of the window and mark the screw holes.
Install the Lock
Use your drill to make holes where you have placed pencil marks. Put the lock pieces in the appropriate spots on the window sash and screw them in place using your screwdriver. Test the lock to make sure it locks securely.