This is the standard route up Whatcom from Perfect Pass, which is also the normal jumping-off point for parties heading up Challenger. Whatcom is a good warm-up climb or half-day outing, with easy snow to access the ridge and fantastic views.
Climb north from Perfect Pass to access the Whatcom glacier/snowfield. Climb the snowfield, aiming for the obvious col in the summit ridge. The ridge is accessed either from the main col directly, or via ramps and ledges slightly climbers right (northeast) of the col.
North of Perfect Pass. This route is normally accessed from Perfect Pass, which in turn is accessed by hiking from the Hannegan Pass trailhead (near Mt. Shuksan), down the Chilliwack River drainage, and then up along Easy Ridge. The approach takes most parties 2 days, and significant navigational challenges may exist. See various references for Picket Range approaches for additional beta.
We found that the last ~100 feet required protection due to exposure and poor rock. A light trad rack would suffice, and opportunities for protection are limited.