Follow the directions for the regular trail of Mt Whitney. After the junction with the John Muir trail, continue up the Whitney trail for about 500m to a wide sandy gulley. There is nothing to mark the spot but you should see a use-trail. Mt Muir is only about 100m higher than the trail, but those are almost vertical meters. If you find the easiest route, it will be all class-3 climbing with scary exposure. The trick is to find the crack/chimney that helps bypass some colossal boulders.