Fairly straightforward - if you compress the self adjusting spring you should get some slack in the cable. Otherwise cut the old cable off. Beware that if you do this the metal finger and rubber buffer will fall. Assuming you have compressed the spring then on the release fork by the gearbox - remove the small metal finger under the rubber buffer.
Working inside the car - remove the drivers side low dash tray and then unhook the cable from the pedal. Back in the engine bay - remove the plastic scuttle panel. Then pull the cable through the bulkhead and off the release fork.
Refitting - keep the new selfadjusting spring compressed (it should already be). Feed the cable through the fork, buffer and slide the metal finger back in. Pass the cable back through the bulkhead and attach the grommet. Inside the car - hook the cable to the pedal.
There should now be some tension - remove the spring retainer and pump the pedal 10 -15 times.
Refit the lower dash tray, scuttle panel, clear up and take the car for a spin.
Most mk2 GTi owners go for the less troublesome manual adjust cable.