Scramble up Diamond PeakWe started on the Pacific Crest Trail, that skirts near the SW corner of Summit Lake. The route up to the South Ridge off of the PCT was not very well marked. Once on top, the trail is fairly easy to the false summit and summit. We left the PCT and used a route maked by a cairn going up but found a better route coming down that is about 50 feet past the hairpin "lookout" on the PCT. That trail was marked by red tape flags, and appeared to be the best route up to the ridge. Beware. There are lots of rock cairns all over, but some of them are not the best route.
One other note - we found a very nice campground on the west shore of Summit Lake. Drive past all of the campers and keep going until you find an empty spot. Free camping and no wilderness charge. But bring a pen and piece of paper, as the wilderness permit stand was out of tags, so we had to make our own.