Understand that smoke and heat from fires rises. Therefore you want to place smoke detectors up high, either on a ceiling or at the top of a wall, no more than 12 inches below the ceiling.
Determine whether or not the home needing smoke detectors is a ranch or a multi-level house. Multi-level homes will require smoke detectors on all levels of the house.
Place a smoke detector within 15 feet of the house's bedrooms. This way the tenenants of the house will be awakened even if the door is closed. If there are particularly sound sleepers, in the house they should be placed even closer to the door.
Avoid placing smoke detectors within three feet of any air registers that may suck smoke away from the smoke detector, rendering it useless.