For reasons that continue to elude me, being generally incapable of using any tool more sophisticated than a screwdriver, I decided to take on the task of replacing a regular doorknob set with a lock-and-key doorknob set.
I was very impressed with myself that I was able to figure out how the assembly worked during the process of taking apart the old knob. And despite having two false starts*, I was able to get the whole thing put together and working in 25 minutes. And I never even read the directions!
I think I may retire while I’m on top of my game. 
*The first time I put the knobs together, I realized that it was OK, but that the lock at a 45-degree angle. Sure, it would have worked, but that would have bugged me endlessly. The second time, I put the knobs together correctly, then I screwed on the…thingy…to the door frame and tried to close the door, only to discover that the latching mechanism was on backwards. Oops. Third time’s a charm!