Our route combined a mix of trails and cross-country travel, and roughly followed the path shown on the Traditional Mountaineering website. In summary, we started at the Devil’s Lake trailhead and hiked across the Wikiup Plain to the Pacific Crest Trail (PCT). We traveled north on the PCT until Separation Creek; from here, we trekked cross-country upstream to the ridge connecting South Sister and Middle Sister. We traversed over the moraines surrounding Chambers Lakes, and descended on a rough climbers trail to Camp Lake. We bivvy-camped on a ridge above Camp Lake. In the morning, we traversed cross-country along the east shoulders of South Sister and joined the trail north of Green Lakes. After a short break at Green Lakes, we followed the trail to the Green Lakes trailhead and then walked along the road to return to our car at the Devil’s Lake trailead.
Overall, the route was straightforward. The cross-country section south of Camp Lake provided some (fun) navigational challenges, but I don’t think it was particularly dangerous. Here is a map (linked from the Traditional Mountaineering website):