I use several layers of the same or different colors and to finish one black layer with a very short exposure, like 1/2 or 1/3 of the other layers. This way the black isn't omni present, but just enough to emphasis the shadows, giving the print more depth. Registering is done with pins through the negative and paper or with marks on the negative outside of the printing area. Or I make sure the negative is the same size as the paper and I put an arrow on the back, telling me which side is the top, not making that mistake again after coating ;-)