To lock the door from the outside to lock the slide lock from the outside you need to install a lock handle with the spindle inserted into the square hole next to the operating lever.
How to install garage door bolt lock.
Make sure that the there s enough space for the bolt to pass through the rail and for the deadbolt to go on.
Once installed turn the handle 90 degrees to lock the slide lock.
This will turn the operating lever and extend the bolt into the locked position.
Secure the lock to the door and put marks where you will drill the holes.
In either case installing a deadbolt lock into your exterior garage door is probably the most cost effective way of increasing your residential security.