Must have a Base pass.
After entering the South gate on Base .25 of a mile there is a parking lot on your left, you park there then you can unload and road the paved road (Arguello Blvd) until you reach Santa Ynez Ridge Rd, make a left and follow that until you reach the first road on your left, which is very short to the huge golf ball. There is a small building to the right head that way and get a great view of Lompoc Valley. From there you'll see you trail down the hill, it is mostly hard packed dirt which makes a very fast descent. About 3/4 of the way down you'll come upon a fence with a gate, be careful sometimes its open sometimes not, if it is not no worries just stop pick your bike up and over the very short fence and step over it (I am 5'8" and have no problem getting over it).
Once you pass the gate it a short up hill climb, to another short downhill to the creek. Now there is a narrow path down the creek bank and across then it pops out at the end of Clark st, take Clark st back to were you parked about a mile.