On my particular brand of door knob the lock mechanism is only held in by a decorative ring and is easily slid off to remove the lock. Also remember how it came out although it should only fit one way. Once its out, test it and see how it works. Basically the key goes in and each tooth on the key pushes a different pin in the lock to a specified height so that they all align and the barrel can turn freely. there are plenty of sites that explain this much better than I can, How Lock Picking Works
. If you open the link for this step keep it open for the next step too.