Once one end of the warp is secured in a knot, and the other pulled to create tension in the warp (however you want to accomplish this--I use a backstrap technique), you can start worrying about the direction in which the threads come through the cards.
Often, these threading directions are referred to, respectively, S and Z threading. I find these terms counterintuitive, so I don't use them, but many find them helpful.
When the "S" tablet is turned 90º toward the weaver, it creates a stitch that bends to the left. Correspondingly, when the "Z" tablet is also turned a quarter-turn toward the weaver, it will create a stitch that bends to the right.