From the trailhead, head out on Doudy Draw. After a short ride (maybe a mile or a mile-and-a-half), you'll come to a junction with the Spring Brook South Trail. Take a right, and head up to the South / North fork. You can ride this loop in either direction (I prefer counterclockwise, starting with the North fork).
This is a pretty easy ride that's close to Boulder and the southern suburbs. The first part of the trail is a smooth walking path that gets rockier as it climbs toward the trees. Even the rockiest parts of the trail aren't very technically challenging, and the climb is pretty gentle. Once you reach the far side of the loop, you start descending and can coast most of the way back down to the trailhead.