The Pacific Crest Trail can be reached from several points in Castle Crags State Park:
The Crags Trail begins at the Vista Point parking area, reached by a steep, winding, narrow paved road from the Park Headquarters. This trailhead has a bathroom. Hike west along a level trail until a sign directs you to the left for the "Crags Trail". A short uphill section brings you to a powerline right-of-way where you meet the PCT.
The Bob's Hat Trail begins on the entrance road just past the Headquarters building. Park in the lot to the south (with a bathroom) and hike up the road past a metal gate. Soon the road turns to dirt and twists and turns uphill untill you reach a signpost on the right directing you to the right fork which, yes, continues uphill to another signpost on the left, where you leave the road and follow a trail up to the PCT.
The eastern section of the Flume Trail takes off from the Indian Creek Nature Trail Loop, a few hundred yards "counterclockwise" (south) of the upper bridge. Mostly level except for a "bulge" near the middle, it meets up with the "western section" [my term] at the bathroom near the paved road.
The western section of the Flume Trail starts a short distance past the Park Entrance along the paved Castle Creek Road. Park where you see a small sign for the Environmental Campground on the right. Once you start on the trail there'll be a small bathroom, and the trail contours around the hills toward the west and eventually meets the PCT.