From the lower parking lot, go uphill to the trailhead at the upper lot. From here it's an easy trail to follow. Follow Bitterbrush trail uphill through somewhat rough singletrack. Around the 1 mile mark you will head down a valley and up the other side into a field. From here the trail becomes physically and technically difficult. You will have roughly 1/2 mile of steep ascending with very difficult technical sections. 1 3/4 miles you reach the top of the first hill and descend into the valley only to ascend into a valley meadow. The trailis easy and hard packed. Once you reach nelson loop (3.5 miles) take a left for a switchback ascent or a right for a slightly less agressive ascent. Follow Nelson Loop (2.3 miles) but beware of speed reducing ditches that rangers dig out for drainage reasons. Once you reconect with Bitterbrush trail follow it back to your car.
From Boulder, take US 36 / Foothills Hwy. north toward Lyons. Turn left on Hwy 66, then left on Hwy 7 / St. Vrain Rd. in Lyons. Trailhead is on the right.
http://www.singletracks.com/item.php?c=1&i=16