Place a barometer in a location that does not get direct sunlight and that does not have a source of heat, drafts or wind. A sealed-off room with air conditioning is one option.
Because air pressure is basically the same indoors and out, a barometer can be placed inside or outside. For home use, hang a barometer on a wall in a place where people can read it conveniently. In most cases, this is an indoor wall, but outdoor walls work, too.
Calibrate a barometer when you set it up. Use a local weather source to determine the current atmospheric pressure in the area, and adjust the barometer to that pressure.