If you're doing it from underneath you'll have to drop the exhaust from the manifold back, heat shield and drive shaft and take out the slave cylinder. The gearbox mounts and the support for the mounts can be disconnected but you will need to support the engine from pivoting backwards, It cant go too far with the front brace in place.
The gearbox is held on with a ring of torx head bolts which are threaded into the block except for two that go through the starter motor and have nuts on the end. I think there is also one going from the opposite direction under the exhaust manifold which holds the cover plate to the gearbox.

While the gearbox was down you could also replace the crankshaft end seal and gasket and the input, output and selector seals on the gearbox.
I would also say to change item 6 & 7 in this diagram



They only cost about €5 for both and I've broken the plastic pin before with an over aggressive gear change so worth definitely worth doing. There is a fair amount of work involved in the process but most is relatively straight forward. There is always the possibility for shearing and stripping bolts which would consume the most time but other than that if you have the facilities go for it