Classic route through the heart of Alpine Lakes Wilderness in Washington, US. The route can be done either direction (CW or CCW). For the purposes of the FKT must go through Iron Cap peak. Once in the high country there are other routes to traverse between the valleys, but the Iron Cap one seems more popular (and also probably gains more elevation). Besides, you don't want to miss the views from the ridge.
There are two trailheads, one for each valley. You can start at either but must finish at the same to complete the loop.
Route finding: If doing CCW the trail is easy to follow till Chetwoot lake. After that follow roughly the gpx along or on the talus field to the ridge. Then traverse to just below Iron Cap summit, which you can climb through the red rock/dirt (probably why it's got its name) area with minimal scrambling. One at the top head immediately to the north, as the immediate ridge line cliffs out. Roughly follow the tracks and with some light bushwhacking while descending you end up just under Iron Cap from where you can easily continue to Tank Lakes and finish the loop.