I can't give you a step by step as sadly my A2 was recently written off; but I can describe what I did...
Placement of the plug seems best in the top left-hand of the boot (I think that's the correct placement based on the little evidence I could find) it means the plug pops out in the correct direction and it's in the void between the top of the rear wheel-well and the back shelf/seatbelt fittings. I don't think it could be positioned low without getting in the way of the rear shocks
Even still - you'll still have to fiddle a little to find the right spot in the side panel that has enough space for it that doesn't interfere with the rear seatbelt spool and such. There is room but it's definitely quite tight.
Getting the panel out is quite involved (I'm sure there's a PDF guide on here somewhere with details) but you'll need to remove your false floor (if you have it) along with the fittings and pull up the carpet. Be careful and methodical - bagging screws and fixings with labels is always a good idea so you can put them all back in the right place.
I measured and made a paper template which corresponded to the shape of inner edge of the fitting and stuck it onto the back of the panel with double sided tape when I was sure it was well placed, then went at it with a drill and Dremel. First drilling four holes near the corners (ideally with a bigger bit that matches the radius of the fitting) , then joining the holes with a cutting disk to cut the main meat out of the hole, then used a sanding cylinder to get to the edges and tidy up the radiused corners. The carpet on the visible-boot side may get a little rough and/or melty with the heat from the sanding cylinder but the fitting has about 3-4mm of flange to cover up any minor messiness.
I wired mine direct to the battery - partly so I could use it when the ignition was off, but mostly because I have super-rudimentary electrical skills and didn't want to bork anything by trying to splice it into other cables. I did a few cool things to the cable rather than just it literally being two wires going direct to the battery. One was make sure the cable was heavy-duty enough to inspire confidence that it could handle the power (Can't remember the exact gauge, but don't use doorbell wire). I also wired in a nylon male/female cable connecter/separator (from the battery of an R/C car) so that I could disconnect the cable easily if I wanted to without having to fiddle with the battery terminals. For safety I put an inline fuse (someone should be able to advise on the best ampage - I don't recall but it was low for peace of mind) on to the positive cable, and then soldered (or crimped) some spade or ring connecters to the end. It needed to be fairly long to go under the carpet and under the trim panel. I put the fuse and connector near the spades at the battery end, so it was all easily accessed from within the battery compartment and not hidden somewhere behind the side trim
Then I wired it all up (testing it before refitting the panel/carpet etc).
Oh noes - it didn't work. Oh. Put the fuse in.
It worked!.
Then I put the the panel back in, carpet, false floor etc. It was tricky getting the panel back exactly as before - I recall some issues with some polystyrene packing around the seatbelt reel, and getting it all neatly back under the bootlid shut rubber trim was a little bit taxing, but it wasn't too hard in the end and it all went back where is should with no squeaks or spare fixings.
In total it took me about 2-3 hours to fit. Sourcing the right cable bits and soldering them all together beforehand a little longer.
In my case it worked exactly as intended but I never really used it - I intended to get a little boot fridge but didn't quite get to it The Apple TV media server thing never quite worked as I hoped. It took a while to boot, and my other plan of getting it working with a 12v ->5v step-down converter instead of a 240v power inverter never quite worked (something to do with current and no-name components bought on EchoBay I'm sure)
Good luck with your project - I'm sure it'll look sweet and work well.
Others please chime in with better ways to wire it in so it's part of the main ignition power as I didn't do this making it a bit of a hack electrically (no safety net if you leave something powered with the ignition off. It WILL vampire all your power and flat the battery if you forget about it this way).