They should be three feet from any object, only use one at a time, and plug the heater directly into an outlet. Do not use an extension cord.
"You want to make sure any space heater you do use is U of l tested," said Jesse Marx of German Township Fire Department. "So, United Laboratories test. They're pretty much the founders of making sure everything is safe for use."
Experts also recommend putting it on a hard surface as carpet can be more flammable.