From interstate 10 at Redlands take the Highway 38 exit (Orange Street or University Avenue) north to Hwy 38 and turn right. Drive northeast for about 14 miles to the intersection with Valley of the Falls Road. Bear right and continue 4.5 miles to the Falls Recreation Area, past the town of Forest Falls. Drive to the parking area at the end of the road and walk uphill through the picnic area on the dirt road to reach the trailhead.
7.8 miles (Gary Suttle's book lists it as 8.4 miles) to the summit with 5,419 feet of elevation gain. The trail leaves the upper end of the picnic area and follows a dirt road for aprox 1/2 mile. At a signed area you will cross Mill Creek Wash and start on a steep mile-long climb to a beautifull hanging valley. Vivian Creek camp is located here. The trail follows Vivian Creek for aprox. a mile and a half before reaching Halfway Camp at 8,100 feet. From here the trail switchbacks up aprox. 2.3 miles to High Creek Camp at about 9,200 feet. Just after you cross the creek in High Creek Camp you will start a series of switchbacks up the west facing slope in front of you to a 10,000+ foot ridge where you can see many miles out towards the Baning Pass and the San Jacinto Mountains. The trail/route from High Creek Camp can become unclear if covered in snow. Basically, once you cross the creek you can head east or northest directly up to the ridge. There are 2 main gullies, both of which top out on the ridge. Once the ridge is gained you will head north another mile or two. Soon the summit can be seen in front of you. At about the tree-line a long monotonous northeast uphill traverse starts and brings you to the trail junction of the Dollar Lake Trail and another 1/4 mile the junction of the Fish Creek Trail. Another easy 1/2 mile and you are at the summit pile of rocks, the highest point in Southern California.
It would be helpful if you could note the relevant USGS 7.5-minute quads in the article: Forest Falls and San Gorgonio Mountain.
Pleanty of water. In winter bring snowshoes depending on how much snow has accumulated. Crampons and ice axe may also be needed depending upon conditions. In spring and fall the trail can be very icy. (I've been up there when the entire upper mountain is frozen solid)