This confused me a little. Once you have built a Police Station, select it (or the icon on the right menu). You'll need to own 5 estates or more.
Now click on an estate on the mini-map in the top-right and click
to set a police officer patrolling it. It will change to a green color, indicating an officer in on partol. Select a different estate and allocate another officer. Repeat 3 more times.
This can be set up to unlock the achievement then cancelled, but if you want to keep it you need to constantly produce police cadets, which can be done with Level 2 tenants (Punks).