Good preparations for ......
Doing Hot Rods and other types of auto work since I began helping hand tools to my Dad around the age of ten has provided me with some valuable experience some of which was gained the "hard way" has made a believer out of me. Do it right and prepare for the worst.
As a recommendation, first get the car off the ground with adequate blockage (jack stands) that will withstand the weight of the vehicle. Make sure the ground or floor is solid, if not, make solid "before" you start raising the vehicle. Try to shake the car a bit to see if it is in fact on solid footing.
The height depends upon the room needed to work and the clearance needed to remove and replace the transmission. Prior comments have mentioned, the removal of the tranny and the engine as a unit, if so, more room/clearance will be required than a simple removal of the tranny.
I would highly recommend pulling both together. As Paul A has suggested it is much easier to take apart and "more accurately" done on the assembly if done out of the vehicle.
One of my High School Buddies that moved to Ft Worth, Tx after a stint in the USAF liked to build VW Dune Buggies. He is no more because one fell on him and crushed his chest. All because he had gotten carless with/without the Jack stands.
I can't place enough caution on being careful
And that is my 2cents worth.