This assumes that you do not have a
fuel injected Volkswagen. This also assumes that you have a stock carburetor (I've seen
Solex Pict 30/31 or 34, but I'm sure there are others). All directional instructions (right, left, etc) assume that you are facing the car from the rear.
There are two ways to adjust the idle:
Number 1: The Idle Screw:
On the left side of the carburetor there is a large lever with a wire connected to the bottom and a screw on the top. This is the throttle lever. The wire connects directly to the gas pedal and as it moves it opens and closes the butterfly valve in the carburetor and thusly accelerating or decelerating respectively. The screw on the top dictates how far the butterfly valve will be open when the wire isn't being pulled upon (i.e when the car is idling).
To adjust this simply turn the screw while the car is idling, you'll hear the engine slow down and speed up (clockwise speeds up, counterclockwise down).
Number 2: The Air Idle Screw:
On the left side of the carburetor there will (probably) be a large screw and a small screw somewhere near the center of the carburetor. The large screw is the one we're interested in. This adjusts the amount of fuel mixed with air at idle. Screw it clockwise to lower the amount of fuel, counterclockwise to increase it. Do this while the engine idles, you'll be able to hear the difference.