it is fairly straightforward but hard work...
first find and stop the leaks... make the car watertight as you don't want to do this job twice
cleaning a grotty interior is entirely possible... you just need time, elbow grease and preferably a warm dry sunny day or two - i will be doing this again come spring by the looks of the cabby

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seats: once you have got them out they will be easy to clean...
you need a George vacuum cleaner (or a wet vac and a spray bottle) and some Autosmart (AS) G101 and some AS Biobrisk
spray the seats down with a mixture of the biobrisk 4:1 (iirc) and G101 10:1 combined and then work it using a stiff brush (i use a vikan nail brush) - then extract the mixture using the vac
you can see the results achieved with this method here:
http://www.golfgtiforum.co.uk/index.php?topic=89507.msg1186179#msg1186179carpet: if you can remove from the car it will be MUCH easier....
again.. just about the same procedure as the seats... except I used a pressure washer with a 'blade' nozzle to wash the carpet down section by section once I had sprayed on the G101/Biobrisk and worked it with a brush - I then hung the carpet out to dry once I worked on my interior... you could do this in an airing cupboard/garage/hallway
you can see the results achieved with this method here:
http://www.golfgtiforum.co.uk/index.php?topic=119235.msg1160196#msg1160196as for the dash/remainder of the interior: I use a AS G101 mix at 10:1 and then spray this on using a spray bottle and wipe off using a microfibre. for crevices I use a vacuum and a paintbrush for working into those tight spaces.
hth