Starting from the rock dome at the top, descend Nimby briefly before almost immediately encountering the first sign (on your right) for BOB. Veer right at the sign and begin the steep decent.
The first hundred feet or so are among aspen trees. After the first big roll-down (a very long, steep, rock slab) the trail drops quickly onto a more exposed, rocky ridge. This lower portion of the track is very rocky. The trail continues down the edge of a mild rock ridge as it quickly eats up the remaining elevation before gently spitting you out on the shoulder of Skyline Parkway (paved road).
From the bottom, your best option back up to the top is the Skyline Trail.
NOTE - winter riding: many of the most difficult trails at Piedmont are much "lower-consequence" in the winter and may be doable on an average fat-bike with less risk than summertime riding. However, these trails are rarely packed/ridden, so watch for good weather conditions resulting in packed (but not icy) snow, or hit the trail with some snowshoes the day before your ride.