The Corral Spring Trail starts near the trailhead at Indian Valley school in Walnut Creek. From the trailhead turn left (this is still Briones to Mt. Diablo Trail
). In a few hundred yards you'll come to a paved street. Take a right and the trail will immediately be on your left.
The trail slowly climbs up the side of Shell Ridge eventually running into Costonoan Trail
. It's a well maintained fire road.