Mark 1 GS guide, but same engine as the MK2 So added here.
Disclaimer: As always, this is a guide only and I accept no responsibility for any damage or injury that maybe sustained through following these guidelines.
Tools required:
1. Breaker Bar/Ratchet handle
2. 14mm Socket
3. 14mm Spanner
1. This is the belt we are going to change
2. This is the replacement belt
2. This is the tensioner pulley with the nut arrowed that is used to pull the tensioner back against its spring.
3. I find the best tool for the backing off the tensioner to slacken the belt for removal is a breaker bar with a 14mm socket. You can use a ratchet handle and socket, or even a 14mm spanner, but for leverage, the breaker is much easier. Pull the breaker in a clockwise direction as shown.
4. Slip the belt off the tensioner
5. Once off the idler, you can release the tensioner back onto its spring. Now you need to unthread the old belt from all of the pulleys and over the fan blades. Youc an remove the blades from the viscous coupling first but there is no need.
6. Once my belt was off, it was worse than I thought and split in places. No wonder it was so noisy 
7. Now do the reverse and thread the new belt over the fan blades and re-route it over the pulleys. When putting it back on, I found it easiest to put it around all of the pulleys, including the idler, but leave it off the power steering pump pulley. Below is a diagram for the correct routing of the belt around the pulleys.
8. Now pull the tensioner against its spring again and pull the belt onto the power steering pulley. Release the tensioner.
At this stage, make sure the belt is seated properly on all pulleys. Note the pic below the bottom pulley is out by one tooth and has ridden up on the endge of the pulley. If you find this, take up the tensioner again and re-adjust the seating on the pulley.
9. Once you are happy, remove all tools from the engine bay and start the car, then listen to your new silent belt