To set up a police force, you must first allocate at least 10 Research Points toward Crime Prevention. When this is complete, your scientists will have decided on an appropriate hull for use as a police cruiser. If this hull is already in service, you may immediately begin researching blueprints for the police cruiser. If it isn't, your Crime Prevention research automatically generates 5RP toward its development. The police cruiser is considered a variant on that hull, and is developed under the variant rules.
Once you have the police cruiser design, you must build 5, and assign a Construction Dock (any size) to the police force. These units will be under automatic control, and the dock will be modified to produce only police cruisers. From here on, the police force will take care of itself in all respects, and will be fully functional in two turns.