From the parking spot at the base, ride up for 2 miles. By doing this, you'll get an idea of how the trails flows. The bottom half isn't too bad, the banks could be taller and there are a few bad rocks, but the second half is steep. It drops 500 ft in 1 mile and is filled with switchbacks that are barely wider than your bike. At the top, you'll see a giant sign and the Spanish Springs Canyon Singletrack Loop. If you go left from the sign, it is made from rocks. Literally the trail is made of rocks. We stopped about 1 mile out and turned around. To the right it is smoother, but we haven't ridden far enough to really know. Anyways, going down the zipper is where it really gets fun. You can make it all the way down to the bottom in less than 10 minutes, but just be careful with the switchbacks and the steep parts.
On I-80 headed whatever direction get off at sparks blvd. headed north. Follow it a long ways out until you get to the Spanish Springs windmill. Then keep following it. Go past the starcrest sign and on your next right you should see a big park being built. Follow the BLM signs all the way to the back of the park. Follow the dirt road for around 1/2 a mile and you should see the singletrack.
http://www.singletracks.com/item.php?c=1&i=2396