Follow the Ruby Crest Trail to Favre Lake. From Favre Lake follow the drainage or use trails up to Castle Lake. Go around the norheast side of Castle Lake and head up the slopes to the base of the couloir. As you leave Liberty Lake and continually on your approach to Castle Lake, take good looks at the face and identify landmarks (e.g. snowfield, patch of trees) to help you find the bottom of the couloir. It will look different as you get closer and move around Castle Lake.
Once you get into the couloir (see pictures), routefinding is not an issue. From the exit of the couloir head east below the ridgeline to the final summit slope. For an easier descent than retracing your way down the couloir, head southwest from the summit down the north slope and pick up the Ruby Crest Trail which you can follow back to Favre and Liberty lakes.
Due to loose talus and propensity for generating rockfall, it is best not to follow someone directly. Try to stay parallel to one another as you ascend so you don't kick rocks down on the person following you. There are a couple narrow sections of the couloir where it is advisable to let the leader proceed alone while the follower(s) stay in a safe location out of the fall line.
For a late summer ascent, no special gear is required. The couloir is filled with loose talus though, so I recommend boots high enough to protect your ankles from rolled over rocks.
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