Move up the right side of the Arch to the right side of the roof. Climb the short crack through the overhang (crux), then go right to a belay/rappel tree or go up to the GT ledge. This is fun if you climb Arch to the crux instead of using the dirty and vegetated corner. This wouldn't be a bad choice for a someone who wants to start leading 5.9. It's strenuous in nature but very well protected.
There are two more pitches listed in the Swain guide; the 2004 Williams guide only describes the single pitch I've written up here.
Location
The easiest way to find this if you are not well acquainted with the cliff is to go up the trail that is across from where the Stairmaster meets the carriage road; there is a Mohonk Preserve sign there. AT the cliff, turn left and walk 100 feet or so. The Arch is just around the corner from a prominent arete.