Follow the track located on the Humbug Hurry-up website http://www.humbughurryup.com. There are a lot of single track and double track on this course. This trail is just plain fun. The beginner course is on city land while the more difficult courses run through private property. Well maintained, easy access, and there is a bike shop on Greenhorn Rd right before the park!
You don't need the official trail map. Instead, if you want just a fun ride, find the start and be spontaneous! Some highlights are riding on a wooden bridge/walkway near the lake, view from the water tank, and the miles of nice trails through the woods.
I5 to the town of Yreka. Take Greenhorn Rd toward the reservoir (Greenhorn Park). Make a left after the reservoir and park in the parking lot. The trail starts south of the lawn that faces away from the reservoir. If you cross the bridge to the south of the lawn, you went to far.
http://www.singletracks.com/item.php?c=1&i=3845