The user manual explains how to wire up solenoids:
simply put, there is a common wire, and then each zone has its individual zone wire.
Just to make sure: you said your solenoids are 12VDC. I assume you have OpenSprinkler 2.3 DC-powered (if you have AC-powered OpenSprinkler, that won’t work with DC valves). If your solenoid has polarity, the positive should go to common, negative should go to individual zone port. Most solenoids (except latching ones) don’t have polarity so most likely there is no distinction between positive vs. negative.