I did a write up for my forthcoming readers ride (one day ill take pics and put it up). Ill summarise it here.
Its pretty simple put theres a couple of fiddly bits and a bit of cutting. Bear in mind that mine was an OEM Golf MKIV one, the retrofit kits you get may be slightly different. Its also worth noting, correct me if I am wrong but I saw in a post here a few weeks back that someone got a Seat OEM one and it had a rectangular lid like the A3 ones rather than rounded front of the Golf ones. So if your worried about that kind of thing an A3/4 one might be better than a Golf one (they are all the same fitting).
As everyone else has said you take out all the console bolts, i didnt bother taking the gear knob and TC fuel flap buttons off as just having the console slack gives you enough room, but by all means remove them and take the console out if you want.
You said you got a kit on fleabay, I assume that comes with all the bolts required? you need an adaptor bolt that screws into where the handbreak is mounted, you then mount the shortest "leg" of the armrest bracket on top of the adaptor. Screwing in the other bolts is a bit fiddly and requires patience and three hands, although again if you have the complete set it may be easier (I got my armrest from the scrap yard, got an adaptor from the stealer and used bolts I had at home). Its pretty obvious where the bolts go, you cant go wrong really just a bit fiddly.
After that you are ready to cut the console. There are two ways to go about this. You can either cut the bottom of the coin tray out, then cut the side bits of the armrest down (the plastic cover with the round bits) so they come down flush with the console. OR you can cut a bigger hole in the console (bottom and sides of the tray) so that the whole thing fits into the hole you have made.
I did it the first way, you will see a cut edge either way but I thought that doing it this way means that if the armrest was ever removed then the console could be repaired easily with one of those kits you get, it is also far cheaper to buy a new side bit for the armrest than it is for a console if you make a mess of cutting it
! If you do it this way then you will notice there are three clips that hold the sides of the armrest together, you want to cut it just on the bottom clip so that its the right length.
After that put the console back on, bolt the armrest itself onto the bracket and give everything a test fit. If its all good put all the TC buttons and that back in (if you took them out) and bobs your uncle.
Take your time with the cutting and try to do it straight, you can always file rough bits down. If you wanted to you could always go round the hole with some rubber beading afterwards to tidy it up. If you cut it straight and neat it will look fine though.
Enjoy your armrest, without a doubt the best thing I have put on my car!