How do I shelter in place?
If you are in a building, close all the doors and windows. Make sure they are latched so you have the best seal possible. Turn your furnace fan off, especially if it draws air from the outside. Even if it doesn't, turn the fan off unless there are extreme temperatures. Dead air is better than moving air.
Turn off all the ventilation fans in the home or building, including bathroom fans, stove fans, and even power attic fans. Shut off the clothes dryer. Close any dampers on fireplaces or wood stoves. If there is a fire burning, let it burn down. If you're using a wood stove, close the fresh air intakes.
Select a shelter room
After you have sealed up the house as well as possible, you can help yourself even more by picking one room in the house to use as a shelter room. Pick a room on your highest level. Most of chemicals that are of concern are heavier than air and will settle in your basement. Try to pick a room that has few or no windows. It also helps if it is on the leeward (downwind) side of the house. A bathroom is usually a good choice because you will have water and a toilet if needed. A master bedroom is also a good choice if it has a bathroom and a phone. Once you have picked a room, bring everyone inside, including your pets. Bring some water (if needed) and a few snacks. Make sure you have a battery-powered radio and a flashlight or two in case the power goes out. Bring some toys if you have small children.
Inside the room
Once in the room, you can seal windows with plastic sheeting and duct tape. The air vents and bathroom fan should be sealed. In some homes, light switches and electrical outlets on outside walls are sources of air infiltration and should also be sealed with duct tape and plastic. Finally, seal the area around the door with duct tape. If the space under the door is too big to seal with tape, try stuffing a damp towel under the door.
Remember, even if you can smell the chemical in your safe room or car, don't panic. You are still being exposed to lower levels of the chemical than if you were outside. Media may provide information on dealing with low levels of that particular chemical as recommended by local officials.
Constantly monitor the radio or television news in case circumstances change. Dont' call 911 unless you have an emergency like a fire or a serious injury. Keep your phone available in case someone needs to contact you. Keep one non-cordless phone in the house in case the electricity goes out.