The instructions for this are rather convoluted, as I recall. The simple part is to flip the MLU when you are set, then take your shot. (it's the small lever on the lower front of the body) The
important thing is to move the lever back
before winding on to the next frame. If you make the mistake of forgetting this, you do not have to waste a frame. First, put your lens cap on. Then set the GS-1 to multiple exposure. Then fire your shutter. Reset your MLU, turn off your multiple exposre and all will be well. Surpisingly this is one mistake I have not made, though I did once incomprehensibly set my MLU while shooting handheld. (whatever was I thinking

)
Enjoy your GS-1. A couple of words of advice... keep an eye on your darkslides, and consider picking up extra backs if you can find them. I had a minor issue with my only GS-1 back while on vacation in South America, while shooting my second roll of the trip. Minor, but it could not be repaired until I returned to Canada. An inoperable GS-1 is something of a brick to be carrying around.
Cheers,