The engine and transmission are really supported at two points - two mounts underneath the engine and one under the back of the transmission. So if you take the transmission mount off without supporting the transmission...yeah.
You get the idea.
I would recommend removing the entire exhaust system, up to the muffler, in one piece as a start. From there you just have to support the transmission and remove the crossmember and mount. Bolt in the new one and put everything back together. If it's your first time, I'd plan for it to take a few hours. I think I could get it done in well under an hour at this point.
The exhaust system is heavy, though. Either be really careful and have it supported when you remove it or have a friend on hand. That way your friend gets crushed instead of you.
By the way, this post belongs in the Tech Q&A forum. Please post in the appropriate forums in the future.