Follow the directions in the "Getting There" section of main page for directions to the Old Fall River Road. This route starts at about 8,800ft. at the Endovalley picnic area.
This is not a marked trail but the route is mostly treeless and on scree. The route follows the Sundance Creek up the gully known as Hanging Valley to the east side of Sundance Mountain. From the Endovalley picnic area at 8,800ft. follow the creek until it creates an intersection with another creek that comes down the mountain from the south. This is your route to follow. Head southwest along the creek until you get out of the trees. Continue up the shallow valley following the creek to it's starting point almost on top of Trail Ridge. This is about a mile and three quarters from your starting point. From hear head west along the ridgeline for about a half mile to a pretty flat area on the ridge. From here you should be able to see the auto-road and the summit of Sundance Mountain. From here you can see your route along the ridgeline heading west up to the summit. The total elevation gain is 3,700ft. and is a round-trip distance of about 9 miles if you descend the way you came.
A daypack and some trekking poles would be plenty for a summer hike up to the summit of Sundance. Snow however does linger in the gullys and colouirs long into the summer so snow gear such as crampoons and an ice axe may be needed. In all winter conditions consider the route technical and use all winter gear.