Step 2: A single point. A single perspective.
One-point perspective is marked by the fact that all objects seem to converge towards one solitary point on the horizon. In order for all objects to converge at a single point, their closest face has to appear to be parallel to horizon.
In other words, if there was a cube between you and the horizon, the face of the cube closest to you would have two horizontal lines parallel to the horizon. In fact, everything viewed in this perspective must have horizontal lines parallel to the horizon.
If horizontal lines are no longer parallel, you have just gained a whole new perspective (but lets not worry about that for now).