Sheltering-in-place simply refers to taking refuge in a building, in the event there is an airborne hazard, rather than evacuating from an area. Depending on the situation, it may be safer to quickly shelter-in-place rather than try to drive to a safe area. It might be sheltering at home, at work or in some other other building and if you understand a few principles, it should be easy.

The purpose is to get out of the outside-air and inside a building that is not contaminated. You want to keep the inside-air in, and the outside-air out, until the bad outside-air blows away. 

When sheltering-in-place:

1. Close all windows and doors, exterior and interior. Open interior doors as little as possible.
2. Turn off all fans, heaters and air conditioners so that they don’t continue to blow outside-air into your house.
3. Close the fireplace damper so that outside-air can’t blow down the chimney and inside the building.
4. Take water, food and disaster supplies, including a radio, into the most airtight room, preferably one without windows, but with a phone. Bring your pets in with you, along with additional water and food for them.
5. Use duct tape or other strong tape to seal all cracks around the door, vents, etc., into the room to help keep out any outside-air.
6. Listen on your radio or television for further instructions.

Read this article for further information: https://www.redcross.org/get-help/how-to-prepare-for-emergencies.html