Firstly, jacking up the car makes it unstable, now matter how you do it.
Only use the supplied scissor jack for emergency wheel changing, in the event of puncture. DO NOT USE this jack if you intend getting under the car.
I use a hydraulic trolley jack to lift the car.
THIS ONE
The car should be supported on axle stands or proper ramps, with the wheels still on the ground being chocked to prevent possible movement.
If intending to work under the car, jack up the car on a solid flat surface, and support the car securely.
To sum up - use a jack only to raise the car, support it on substantial axle stands/ramps. Chock the grounded wheels to prevent movement.
If possible leave the car in gear with the handbrake applied. (Obviously this is dependent on what you are trying to do on the car.)
Having said that these are the jacking points.
My Mk2 F has an additional, very handy central jacking point at the rear too, highlighted in orange here.
I only use the 2 central jacking points as each raises one end of the car.
Personally, I would never use the sills to jack up the car, except in dire need.
Clive's car raised on the rear sill jacking point using the MGR scissor jack.
...and now supported by an axle stand.
Clive's car supported by ramps.
Trolley jack used on the central rear jacking point. Two axle stands are under the subframe to support the weight.
Clive's car with the scissor jack on the front sill jack point.
Broon's front wheels supported on ramps.
Bryan Young's Axle Stands under the subframe to allow the front suspension to be stripped.