The "montage" listing shows you which sites on the head go to which inputs in each channel. Example:
You must ALWAYS have a ground. It may not be specified in the Montage.
1. Ground: Place electrode behind the left ear
2. Active: Place electrode at Cz
3. Reference: Place electrode behind right ear
You MUST have a ground (Most amplifiers use one ground for all channels), and the ground can go anywhere on the body (such as the other ear).