How do I top belay?
There are two methods to belay from the top of a pitch. The modern tendency is to place the belay device on the anchor directly, but there are cases where belaying off your harness and redirecting the climbing rope upwards to an anchor makes sense.
To belay off the anchor when using a tubular device with "guide mode", place the device on the anchor with a locking carabiner according to the device instructions. With a ATC-guide, Reverso, or similar device, use the large diameter hole on one end of the aluminum plate. Don't hang the device by the wire keeper loop. Once that's rigged, feed a bight of the climbing rope into the plate per usual (with the rope descending to the climber fed through the upper hole) and capture the loop with another locking carabiner.
To belay off the anchor with a Gri-Gri (or similar auto-assist device), begin by hanging the opened Gri-Gri on the anchor masterpoint via a locking carabiner. Then feed a bight of rope into the device per usual, ensuring that the climber's strand is fed into corresponding indication on the Gri-Gri. Close the device by capturing the other side of the device into the carabiner. Ensure that the handle and cam of the Gri-Gri is not making contact with the rock and, in the course of a fall, cannot be held open by other parts of the rigging.
To belay off your harness, set the climbing rope into your belay device as you would for bottom belaying and then redirect the load strand into the anchor or a bomber piece above your waist. This orients the rope properly for a comfortable belay and prevents the climber from weighting your harness awkwardly and dangerously in the case of a fall.
In every method, test your rigging by pulling hard on the climber's strand and making sure that the device locks up.