In short, do exactly what's described, here...
http://stasf2008.ephpbb.com/t6-steel-tank-add-on-steel-fury-1-2...and here...
http://stasf2008.ephpbb.com/t20-how-to-install-sta-modIIRC you need to register on the forum, for the download links to be active. STA1.2 is the latest mod on general release; STA2 was not universally released, before work switched to STA3, which remains in development.
The 'Game Mission Packs' and the 'Weather [terrain/seasonal) Mods' are optional, but highly recommended, on top of the STA mod. Do not mix STA elements with others like the unofficial patch.
That will get you the latest mod and the playable vehicles illustrated here, and more besides:
http://stasf2008.ephpbb.com/t22-available-units-in-sta-modGood hunting!