The trailhead is along the road just west of the Nixon Ranger Station and near a concrete reservoir below Nixon Spring. It's well-signed.
Follow the trail into the trees as it heads slightly southeast at first. It swings east then starts a series of switchbacks. After these the trail generally makes a long sweeping traverse across the south flanks of the mountain, occasionally switchbacking but usually just going in one direction and often somewhat steeply. The trail is covered in small rubbly lava rocks so watch the footing.
After about 2 miles the trail levels off and makes a noticeable bend northerly (slightly west of north). It crosses a few minor drainages and eventually peters out amid the tall pines of the summit plateau. Keep an eye as you will need to start looking for some cairns. The trail itself can be indistinct amid the carpet of pine needles. Pay attention.
The last 1/2 - 3/4 mile to the top follows cairns and logs that have been moved to delineate the route. You may have to stop and look for the next cairn. Eventually the route crosses a huge downed tree, then starts up one last little hill to top out on the summit. The USGS "Trumbull" BM is found beneath an old wooden triangulation apparatus, and two witness markers are nearby. The summit is wooded but views are pretty nice to the north and to the southeast.
Return the same route and watch for the cairns.
Definitely a map, compass and GPS in case you get off the final bit. Good hiking boots and the usual hiking duds are adequate. The round trip is about 5 miles with about 1,500 feet of gain. Allow about 4 hours.
Nixon Springs Aqueduct
Some books mention hiking N up the aqueduct then cross country to the top. I can't comment on this route. It seems a bit much more work than necessary...