Step 1: Original Lenses at the Rear.
!! SAFETY NOTE !! - Use safety goggles for this step as it is very easy for the drill to slip and break the bit, or for some small part of the screw head to come flying out.
Step 2: Access Inside to the Wiring.
You need to remove the cover by undoing the bolts and screws as indicated by the purple arrows. Check the wiring colours for the rear lamps before you attack anything else.
Take pictures with a digital camera to act as an aid mémoire.
Step 3: Aligning Rear Plinths
More care needs to be taken when placing the rear lenses. I would suggest using a bright marker of some sort and then stepping back from the vehicle to make sure things look ok. There are legal requirements in some countries for the position of rear lamps etc.
Once a position has been chosen, the fitting of the lamps is very similar to the front lamps covered in' Upgrading the exterior lights on a landrover defender to NAS. (Part 2 of 3).'
Step 4: The END
If you made it this far then thanks for reading.
I hope these three small Instructables prove to be of some use.