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.
Once installed turn the handle 90 degrees to lock the slide lock.
If money is an issue as these days it often times is you can perform the installation yourself.
After you have drilled the holes fasten the lock in place with screws.
In either case installing a deadbolt lock into your exterior garage door is probably the most cost effective way of increasing your residential security.
Secure the lock to the door and put marks where you will drill the holes.