Only work on hard, level ground.
Never climb under a vehicle suspended on a jack - always transfer it to jack stands first.
Care should be taken to find a safe part of the vehicle to support its weight upon the stand and the jack, such as K frames, axles and designated jacking points. Placing jacks or stands under a weak part of the vehicle may cause the jack or stand to puncture the underide of the vehicle, causing damage and risking serious injury.
Jacks and stands should be positioned where the vehicle can not slide off the contact point.
When climbing under a vehicle on jack stands, place one or more wheels under it as precaution. In the case of an accident, the vehicle's weight can be supported by the wheel, reducing the severity of the outcome.
Check the weight rating of the jack and jack stands against the weight of your vehicle. You can find your vehicle's weight in the owner's manual or on the vehicle's Australian compliance plate (usually in the engine bay).
On most cars (those with beam axle, solid axle, or trailing arm suspension), the car will move backward as the rear end lifts up, even as the tyres remain still. Therefore to avoid having the jack tip over and drop the vehicle, lift the front of the vehicle before the back.
If you're at all unsure, ask for help on the Automotive Cause Discord channel before using jack stands. Lifting a car incorrectly is very dangerous.