Ah no, I'm not a legend, and in fact I struck it lucky with the first actuator re-fitting, stuffed it up big time the second time because those linkages had dropped off the sensors. So I'm far from perfect at this, and there are always more mistakes to be made and learned from.
Of course, you will have already removed the battery tray (one of the four battery tray bolts is under the plastic tray the battery sits in). The air filter has a (frankly useless) plastic bracket that comes off the front of the battery tray.
By the way, when you take the exhaust system off, the red 'rubber' hanger back from the engine - don't try to unhook it - just unbolt its bracket from the subframe. Much easier!
De-pressurising the Selespeed - no need, after a few hours without the pump running, just undo the two fluid lines (one has a threaded connection: use a spanner to hold the nut that the pipe nut screws into, otherwise the whole shebang unscrews from the actuator and the pipe twists). The other fluid return pipe just has the usual hose clip. Cable-tie the two fluid lines up near the headlight to minimise loss of that expensive Selespeed fluid.
The selector bolt I mentioned before is really hard to see and reach. You probably want to unbolt the front gearbox mounting first, with the engine/transmission supported on a jack, then lower it so that you can see the side of the actuator through the wheelarch.
It is really difficult (well, I thought it was) to remove the gearbox with the subframe and right-side (long) driveshaft still in place, because the diff housing can't get over the subframe. If you jack the engine up high enough for the diff to clear the subframe, the top of the gearbox won't clear the side of the engine bay (at least, not in a 156). For a clutch change, the gearbox can just be moved across far enough and left sitting on the subframe. But to get the gearbox completely out for a repair, it would be a lot easier if the right-side driveshaft could be removed first. Unfortunately the bracket for that driveshaft runs all the way up the back of the engine - it even has the top engine mount attached. I'm not sure how to remove it.
I didn't remove the driveshaft or subframe and I had a titanic struggle that ended with the gearbox falling out at an awful angle, possibly damaging the selector mechanism as the rod snagged the side of the engine bay, and tearing the seal on the diff housing for that driveshaft (had to waste hours getting an expensive replacement of this special-sized seal - something like 27.5mm, definitely ends in .5mm so not a standard size).
Refitting the gearbox was even harder, jacking the engine up and down trying to get both driveshaft and input shaft (clutch) to align but again with the gearbox mashing against aluminium pipes (power steering), selector rod catching, and diff housing rolling/sliding down the slope of the subframe. Imagine a two-hour struggle leaning in over the side wrangling the gearbox around
My back feels sore just remembering it. There has to be a better way...
If taking the right-side driveshaft off doesn't tickle your fancy, perhaps remove the subframe instead (especially if you want to replace your antiroll bar at the same time... worn bushes/clonking?)
With the subframe out of the way, the gearbox would slide straight off, and the right-side driveshaft could stay in place.
Once you have everything back up together, pressurising is automatic. Check for error codes. If there are P1741/P1742 codes, it won't calibrate: that means the position sensors have fallen off the linkage and the actuator has to come off again, back plate has to come off the actuator so that the linkage bits can be put back together. Once there are no codes, then you can set the clutch and EOL calibrate.
I'm fairly certain the worst part of the job will be the removal/refitting of the gearbox - make sure you have at least two trolley jacks and axle stands, and get to work...