The easiest way that I have of making anlged tennons, is to use loose tennons. Just cut your angle as a butt joint, then mortis perpendiculat to that face, and place a mortise in your mating piece. A loose tennon fills the voids, and creates a joint that will outlast you and me...
If you're looking for a little more traditional, just lay it out, and cut it by hand, or with a band saw... It really depends on how many you have to do.... If a single chair, then by hand may be fine... If doing a set of chairs, spend a few minutes on some fixtures/jigs...
Hope this helps