What safety precautions should be taken when using an electric cushion?

It using it as intended by the manufacturer, none. Usually there will be a tab attatched to the product or on the packaging telling you what cannot be done with or to it on penalty of it no longer functioning properly. But most if not all cushions are very low voltage (9 to 12 V) and have a transformator between the cushion and the electricity socket. So they are made to be extremely low-risk. The only safety precaution you will be advised to take is printed on the plastic bag it comes in: "do not pull it over your head as this may lead to suffocation".