What key steps to be undertaken before starting work on a motor vehicle?

What key steps to be undertaken before starting work on a motor vehicle?
It all depends on what part of the vehicle you're going to be working on. Disconnecting the negative battery cable is usually a good idea.

Making sure the vehicle is on level ground, the transmission is in gear (either Park for an automatic or 1st gear for a manual), and the emergency brake is set.

Note the emergency brake only holds the rear wheels, so if you jack up the rear axle the car might roll.... make sure to use wheel chocks on the front wheels to keep this from happening.

Also never go underneath a car which is only supported by a jack. Use the jack to raise the car, then place jack stands and lower the car onto the jack stands.... jacks can fall.