Start at the Lava Lake Trailhead at 5300 feet elevation. The trail goes Southwest from the parking area.
At mile 0.2 is the junction with the Pacific Crest Trail (PCT, #2000). Stay left (South). Right goes to McKenzie Pass
At mile 0.9 (5460') is the junction with the Mattieu Lake Trail #4062. You can go either way - it's the same distance, but for this hike, I arbitrarily say to stay right.
At mile 2.1 (5750') is North Mattieu Lake. There are many nice campsites around the lake. This can be crowded on summer weekends. Somewhat sparse forest of pine trees.
At mile 2.8 (6030') is the junction back to the PCT. A few hundred feet further is South Mattieu Lake. There are a few campsites around the lake. There is a little alpine vegetation. Quite a few people on summer weekends, but beyond here the crowds thin out.
At mile 2.9 (6030') is the junction with the Scott Trail #4068. Stay left, on the Scott Trail.
Somewhere around mile 4.1 (5400') is an unmarked trail to Yapoah Lake, which is about 0.3 miles away. Camping next to the lake.
At mile 4.7 (5300') is the junction with the Green Lake Trail #4070. (At Green Lake, the trail number changes to #17.) Go right (South) on the Green Lake Trail. If you stayed on the Scott Trail, it's about 1.7 miles to the Scott Trailhead.
At mile 6.8 (5600') is Alder Creek. This is a good spot to camp and get drinking water. This is the only drinking water between South Mattieu Lake and the Camp Lake Trail. In September the creek may dry up, but there's still probably water flowing underground - go along the streambed and find a pool, maybe enlarge it a bit and then let the silt settle. Or maybe you're better off going to the Camp Creek Trail stream for better drinking water.
Between here and the Pole Creek Trail is a really nice area. The trail gets up to 6260'. Many great places to camp, but no drinking water. You could bring water from Alder Creek (or Camp Creek if you're going counter-clockwise). Lot's of wide open area you could go off trail on. You could go a long ways up on the mountain. You could probably cross over to Camp Lake or Yapoah Crater.
At mile 11.1 (5900') is the junction with the Pole Creek Trail #4072 which goes left (Northeast) 1.4 miles to a trailhead. Stay right (South) on the Green Lakes Trail.
At mile 11.7 (5760') is the junction with the Camp Lake Trail #4074, which goes right (West). There is a drinking water stream (Soap Creek) and a campsite.
There is an alternate route that saves about 13 miles. It's 7.9 miles to the PCT at Seperation Creek on the other side. High point is 7400'. There's a trail most of the way, but for about a mile it's hard to find the faint route. See Camp Lake from Pole Creek Hike for a more complete description.
This hike stays straight (South) on the Green Lakes Trail.
Between here and Park Meadow the trail crosses a number of small streams which provide good drinking water.
At mile 14.8 (6000') is a lake with campsites.