on top of the nail there is a dark color that run the length of the nail. On the bottom there is a white-ish/clear-ish part that only extends partway down the toe. Clip beneath that making sure you don't clip the white part. That is the quick, it hold the veins that flow blood to the toes. If by chance you do nip too close and it does bleed, use the liquid bandages or some flour to stop the bleeding.
A natural way to do this is to have slate tiles in their enclosure, then you don't have to worry about pesky nail clipping.