When ordered to shelter-in-place:

Gather all residents and pets into the pre-selected room
Close and lock all windows and doors. Seal the opening at the bottom of the door with towels if possible
Turn off heating, ventilating and air conditioning systems (HVAC)
Turn off vent fans and any other device that moves air. An exception to this might be an air purifier incorporating high-efficiency particulate air (HEPA) filters
Seal windows, doors, and vents with pre-cut plastic and duct tape. Tape all edges down all the way around
Account for everyone
Contact family or friends to notify them of the location. After this, do not use the telephone so that resources will be available for use by emergency responders
Monitor radio, TV and internet (if possible) for updates
Remain sheltered in place until told that it is safe or ordered to evacuate