Exit SR410 onto FS 70, and drive till the pavement stops. You'll go by the Greenwater Lakes Trailhead. Continue on FS 70 till FR 7080 which is well marked.
The "trailhead" is where PCT crosses Forrest Road 7080 which is a bit of a rough road, but not bad for most of it. Expect to go by a few groups of people shooting on the climb. If you aren't using a GPS for the trailhead, the trail is not well marked, but you'll come to a Y at a sharp right turn with the left road being a 4WD road that is not gravel. There is a sign that says PCT but you'll need to look for it on the right-hand side of the road just before the right-hand turn.
The trail itself is well groomed mostly shaded, and just a steady climb. It starts with a brief descent, and a climb and descent in the middle of the run. There is a great stop for a break at 3.5 miles in with a beautiful view. During dry summers, make sure to have extra water, or be prepared to search it out and filter from small pools.
For the most part, the trail crawls lazily to Arch Rock. The trail empties out a bit to some nice looking meadows, with some good campsites at about 7 miles in on small side trails.