I'm in the middle of doing this vile job myself.
First off you need to check that it is the alternator; There's loads of stuff on the site about 'parasitic' drains of current etc and you should read them all. And then again it could be a duff battery.
First job is to spend more than 7 quid for a multimeter - buy quality - it won't let you down (personally I found out the hard way!)
Now, I won't say that this is a foolproof test BUT with everything switched off you should be able to read about 12.5 volts across the battery terminals. If you have a face off stereo - take the face off to be sure. Check that the boot light isn't on as well. Now get someone to start the engine and hold the revs at 2000. Measure Volts across the battery. If it measures the same or even decreases chances are that your alternator or its diode are shagged out!. If it registers anything up to about 14.7 Volts you are in good shape!
You need to keep this up for a while because alternators appear to deteriorate and as they warm up the current flow might drop downwards.
HOWEVER if it definitely is the alternator prepare to get wet, oily, frustrated and misled!
I am currently all of those things! - I obtained the manual from AO (thanks guys) and downloaded the 5 pages of diagrams. Now my GTV is 96/97 and I think the manual starts in 98. The diagram for the tensioner is wrong (or really badly drawn for the car) it shows the tensioner belt going over the top of the pulley of the tensioner and shows the central bolt of the tensioner being removed. WRONG!!! Its the one underneath that holds the springing mechanism for the tensioner pulley. Undoing this allows the whole tensioner to be withdrawn through the wheelwell after removing what is termed in the manual as 'the mudflap' - its actually the lower part of the plastic inner wing!! (two bolts, and a push fit plastic nail).
This is just the start of the fun! I am nearly five (yes 5) hours in and have just managed to remove the alternator!
I WILL publish a detailed alternator removal and refit process inc photographs when I'm done and once the wife is speaking to me. Cheers - PenR