Remove the old doorknob by taking out the screws located on either side of the knob. If there are no screws visible, remove the inside door knob first by locating the catch inside a little slot or hole and releasing it with a small flat-head screwdriver or sharp, pointed instrument such as an awl or pin.
Pull the two halves of the doorknob apart and out of the door.
Measure the hole in the door and bolt plate or strike plate with a measuring tape. If necessary, replace the old doorknob until the new one can be installed; go to a local hardware store and purchase a new one.
Test the new lock and keys to make sure everything works before beginning the installation.
Install the new latch and bolt plate on the edge of the door.
Insert the half of the door knob that has the spindle, then slip the other half carefully in place with the screws lined up.
Screw both halves together and test the lock again.