This is considered a "beginning" mountain bike trail in the San Gabriels, where most everything is just up or down. After you get your bike and out of your car, go past the yellow gate, and go down the pavement for just under a mile. You'll pass JPL to your left, and several other pseudo-trails on your left and right that don't go anywhere.
You'll reach a fork in the road, and take the left fork. If you take the right fork, you're on the wrong trail. Continue through the wide double track, and you'll be in the forest in no time. At about 2.5 miles, you'll hit your first creek crossing (there are five). The first three are pretty close together, and the second one is usually the worst. The higher you go, the more technical the trail gets. There is one area after the fourth creek crossing that I call "Satan's rock garden" because of the amount of rocks you have to go over with slick wheels from the creek.
Towards the end, the trail becomes singletrack, but it doesn't last long. You'll reach the sixth creek crossing, and you can continue on, but it's a lot of hike-a-bike from that point on.
Turn aronnd, and enjoy the downhill. Just becareful of all the dogs off-leach (no, they're not supposed to be), the horses, the hikers, and all the kids who leave their bikes in the middle of the trail.
The best part about this trail is it's the only shaded trail in the area, which is really nice on a hot summer day.
Take I-210 towards San Fernando (West coming from Monrovia, East coming from Tujunga), and exit Arroyo Parkway/Windsor Ave. Head north, go past the Jack in the Box, cross Woodbury Ave, go 1 mile and look for the parking lot on your left. Street parking is available on side streets from Windsor Ave. This place fillls up early in the morning, particularly in the hot summers.
http://www.singletracks.com/item.php?c=1&i=872