A sash lock is comprised of two parts, the hook and the latch. The hook is mounted on top of the upper meeting rail and the latch is mounted on top of the lower meeting rail. When the lock is engaged, it should pull the the two sash together creating a tight seal.
Position yourself so you are looking down on the upper meeting rail. You may need a step stool to do this.
Existing holes often don’t align with the holes or the correct position of a new sash lock, and they often don’t contain a enough wood fiber to securely hold the screws. If you reuse existing holes, follow these instructions, but it is often best to secure your lock with fresh wood. Simply move your hardware about a quarter inch away from the existing holes. It won’t be too apparent that it isn’t dead center in the window.
Position the base of the hook so that when the lower sash opens, the lower meeting rail doesn’t come in contact with the the hook – make sure it clears by about 3/32”. Carefully mark the center of each hole location with an awl or pencil. Remove the latch and with a hammer and awl, enlarge your marks to form a pilot hole. (Since window sashes are often made from soft wood, it usually isn’t necessary to drill a pilot hole.) Then, secure the hook with screws.
Next, open the latch part way so the tip of the curved wedge is positioned to enter the center of the hook opening. Make sure the base of the latch is directly in front of and parallel to the base of the hook. This is the location of your latch. Mark the hole locations in the same manner used to locate the hook and similarly secure it to the lower meeting rail.
Tip: When operating a sash always make sure the latch is in the open position or you may damage any muntins running horizontally across the upper sash.
Another Tip: If your meeting rails aren’t flush and you use a shim under your sash lock hook, be sure to use longer screws, otherwise when engaging the latch, you might yank the hook out of its holes.