Trace a line on the onyx where you wish the cut to be made with a pencil and straight edge.
Put on your goggles or other eye protection. Turn on the tile wet saw, and make sure there is plenty of water in the basin below the blade. Cutting through marble and resin can overheat the blade quickly, so plenty of water is crucial.
Line up the area of the onyx you wish to cut with the blade on the wet saw's table. Carefully push the onyx toward the blade. When the blade begins to cut the onyx, do not force the tile into the blade. Instead, let the blade pull the onyx into it while you provide slight pressure and guidance with your hands.
Allow the onyx to dry before installing it.