The following safety points cannot be overemphasized:
Always block the opposite wheel or wheels to keep the vehicle from rolling off the jack.
When raising the front of the vehicle, firmly apply the parking brake.
When the drive wheels are to remain on the ground, leave the vehicle in gear to help prevent it from rolling.
Always use jackstands to support the vehicle when you are working underneath. Place the stands beneath the vehicle's jacking brackets. Before climbing underneath, rock the vehicle a bit to make sure it is firmly supported.