First Steps: Under the dash.
Remove the glove box (4x screws)
Remove the puddle light panel (2x screws)
Remove the Glove box mount (red circles, 2x 8mm nuts)
Remove the windscreen duct (blue circle)
Remove the old duct non-ac duct which goes from the air intake to the blower fan (1x bolt, top left circled red)
This should give you as much access to the foot well as you're likely to need.
While you've got this access I'm told it's worth doing something to the heater resistors if your heater blower doesn't work on all the settings (top right connector on the below picture) - you can remove it by undoing the two screws and then follow someone elses how-to on repairing it,
While I had everything out I upgraded by air intake with the later 'snorkel' to try and stop the footwell filling with water when it rains (Rover part JKA000120) by undoing the screws circles in blue and another 2 in the front bay behind the brake servo/mc. Incidentaly, the snorkal bolts in from the top.
Swap the steering column blank (3 bolts I think)
Trim the carpet back (it's already perforated where it needs to be cut)
Remove the blank for the AC hoses to pass through the bulkhead
Fit the Evaporator unit - make sure you've removed the connector block from the AC pipes (item 8 on parts diagram)
It slides in relatively easily - it goes on the inside of the lip on the blower fan and doesn't form a great seal (that's what the large band clamp is for)
Fit the clamp and mounting nuts (one existing one in top left, the other on the new column blank)
Connect the drain pipe.