Same as a regular BO, but the last stitch is going to appear a row higher than the first one. What I do - with the last loop on the right needle, pick up a stitch in the center of the first bound off st and pass the loop over it. Then bring the tail through and weave in the ends. And yeah, it helps to have a stretchy BO, so use one that Jan suggests, or a needle 2 sizes larger.
sue- knitting heretic