On each level of your home
 Outside and near sleeping areas such as bedrooms
In the area of any major gas-burning appliances such as a furnace or water heater
At least 15 feet from gas-burning appliances (not any closer)
On the wall or ceiling. The weight of CO is very close to that of normal atmosphere, so the mounting height does not matter.

In general, follow the manufacturer's instructions.

Scroll through the floor plan renderings below for examples of CO alarm placement.

One Bedroom unit (not to scale):

Two Bedroom Unit (not to scale):

Two Bedroom Unit with Gas Dryer (not to scale):
Note: If bedroom doors are more than 30 feet apart, two carbon monoxide alarms are required.

Two-story Two Bedroom Unit (not to scale):

Three Bedroom Unit with Gas Boiler In Basement (not to scale):

Three Bedroom Unit with Gas Stove and Fireplace (not to scale):

Efficiency/Studio (not to scale):

Unit with One Bedroom That Has Direct Access to the Kitchen, with Gas Furnace In Basement (not to scale): 

Apartment Containing Gas Fireplace (not to scale):

Cross-Section Showing CO Alarm Placement (not to scale):