One of the "Fifty Classic Climbs of North America," the West Ridge is Forbidden Peak's standard route, combining a snow/ice couloir with several pitches of exposed alpine rock climbing.
Register at the Wilderness Information Center in Marblemount, then drive the Cascade River Road (Forest Road 15) about 21.7 miles to the Boston Basin Trailhead, a pullout just before the Cascade Pass Trailhead at road's end. From the trailhead, follow the old Diamond Mine roadbed for about a mile, then a steep, unmaintained climbers' trail that gains roughly 3,000 feet over the next 2-3 miles into Boston Basin, crossing several streams along the way. High camp is typically made on flats around 6,400 feet below the basin's small glaciers, with alternative bivy sites near the top of the approach couloir. From camp, cross the glacier to a prominent, roughly 500-foot-long couloir at the left edge of Forbidden's south face. Depending on the season this couloir holds continuous snow and ice up to about 50 degrees, or has melted out to a mix of snow and smooth glacier-polished granite; either way it leads to the notch at the base of the West Ridge. From the notch, the climbing begins in earnest: roughly 8 pitches of mid-5th-class granite up the exposed ridge crest, following cracks and ledges with sustained exposure but generally solid rock, to the summit.
Descend by reversing the West Ridge with a combination of downclimbing and rappelling back to the notch, then downclimb or rappel the entrance couloir. Whichever couloir variant was climbed (the main gully or the "Cat Scratch" gullies), expect the descent to take nearly as long as the ascent, and allow extra time if it is still snow-covered, which makes the rappels more awkward.