Easy job mate

First take the dash under tray off, and have a look at what you're dealing with.  It may have just slipped off the end of the accelerator calbe, if so hook it back on and off you go.  If it happens again, you'll need to replace the rubber bush on the accelerator, or you might get away with rotating it round a bit.
If the cable is indeed snapped:
1. Take the plastic rain tray off, a few clips round the edges and it will pop free.  The early ones are made of cheap flimsy plastic and its likly to crack, if so VW will sell you a hard plastic cover, or visit your local scrappy looking for late MK2s

2. Pull the cable from the engine bay, and unhook from the carb
3. Thread new cable in the same way, make sure the grommet is seated in the hole in the firewall properly
4. Hook up the pedal end, then the carb end and adjust so that when the pedal is to the floor the carb is just fully open.  It helps to have a mate sat in the car while you do this
