When working on an electrical furnace fan, note it has sharp blades that can seriously injure you if you are inattentive. Also, because it is connected to wires and the furnace, there is a risk of an electrical shock. Turn off the power to the furnace or the blower fan.
HVAC units are typically on their own 220-volt circuit. The circuit might be labeled on the circuit breaker panel. If you are unsure, set the fan to the always-on position and sequentially flip switches until you find the one that turns it off. If you cannot turn off the whole circuit, turn off power to the fan so you can work on it. Even after this point, still exercise caution when dealing with the electrical components.
Keep your hands and tools dry and only use one hand when possible to touch any wires. Touching a source of potential current with both hands provides the electricity an easy pathway through your heart.
When cleaning the fan, stabilize the blade with one hand while wiping another one with a dry cloth on the flat surface. Stay away from the edge to avoid cutting yourself, and remember that the fan blades spin freely.