[Steve Tinker]

You don't - you need a hydraulic jack to raise the car to the appropriate height and then use jack stands to support the vehicle before you venture underneath.
Once you have the jack / stands / sockets its easy to change the gearbox fluid - but if you don't have the right equipment its very difficult to do it correctly.
IF you are serious about performing your own maintenance, first you must invest in the correct tools, which over the long term is a lot cheaper than getting a workshop to do it .....