From the Incinerator Ridge Parking area, head northwest past a large boulder blocking the trail. Campers often use this area by the boulder as a campsite which can be confusing, but continue on past the boulders, and a wider path shortly narrows down into a nice singletrack trail that's easy to follow.
From here, the trail ascends moderately as it switchbacks up the hillside. This section is fairly negotiable and enjoys some nice vistas once you ascend some larger rock outcroppings. The final push up the trail is a bit steeper. At the top of the trail, you'll pour out onto the Mount Bigelow Road, and you're now at the summit of Mt. Bigelow.
There is a trailhead up top for the Butterfly Trail
or you can descend on the Mount Bigelow dirt road to the highway or you can take Boy Scout Trail and Bear Wallow, ultimately leading you to the Sunset Trail.