How to get there:
Drive Santiago Canyon Road from (Toll road) 241, to locate the beginning of Silverado Canyon Road at location N33.7483, W117.6733. Continue approximately 5 miles up Silverado Canyon Road. You will arrive at a Forest Service parking area (Adventure Pass required). There is a gate here, hopefully open. Continue on the narrow, but paved forest service road for approximately 4 more miles. The road turns right and uphill as well as into dirt. Drive another roughly 4 miles. This road is best suited for 4WD or at least high clearance vehicles.
You are now on a ridge where several (dirt) roads meet. The correct road is the second right. If the gate is closed, then hike from here. This location is at N33.72897, W117.54854, elevation about 1380 meter.
Hike the road from the location above. The road first traverses right towards an antenna. Here, it turns the ridge (left) and traverses the other side to reach the saddle between Modjeska Peak and Santiago Peak (elevation 1500 meter). The road now makes a long ascending traverse right, before a final leg going left to reach the summit. Proceed between a small forest of antennas to locate the signed highest point, conveniently located outside a fenced in area.
I drove from my apartment in Westwood fairly early, but traffic was kind of slow and it took quite some time before I arrived in the general area below the peak. I first tried to access the Holy Jim Trailhead, but the access road was closed by a gate and I needed to turn back. Next, I tried the route described by James Barlow - much better, the gate was open and I drove to a ridge at about 1380 meter before hitting a closed (and locked) gate.
I walked from here to the summit in 1:20, the sun was hot and there was little shade. I spent 10 minutes at the summit before returning. When I got to the point where another road forks off to Modjeska Peak, I decided to include it - this was a 30 minute detour. Finally returning back down to my car, descending in 1:10 if I exclude the detour.
I continued back down the same road, then headed across to Riverside where I got stuck in a major traffic jam trying to go south on I-15. I ended up in a nice motel in the small town of Elsinore.