The standard, least technical way up Buckner Mountain, a long day (or more commonly 2-3 day trip) over Cascade Pass and Sahale Arm before dropping into Horseshoe Basin to reach the peak's southwest slope.
Drive the Cascade River Road to its end at the Cascade Pass Trailhead. Hike the well-graded Cascade Pass Trail about 3.7 miles, climbing roughly 1,800 feet, to Cascade Pass itself (camping is not permitted at the pass due to vegetation restoration; nearby designated camps include Pelton Basin, Basin Creek, and Sahale Glacier Camp). From the pass, take the left fork and climb the Sahale Arm Trail toward Sahale Glacier Camp, around 7,600 feet, roughly 2.2 miles further. From this high camp, drop off the south/east side toward Horseshoe Basin, losing on the order of 1,000-1,800 feet, and traverse the basin below Ripsaw Ridge — a long, brushy, and sometimes snow-covered crossing that takes longer than it looks from a distance. From the basin, ascend Buckner's southwest slope, which looks steep from afar but eases to roughly 30-40 degrees on approach; this is typically snow early in the season and scree or talus by late summer, with some short rock steps (up to Class 4) mixed in. The slope leads to the southwest summit, which is the point most parties reach and is treated as the standard summit even though the northeast summit may be marginally higher (accounts differ, and neither figure is well documented).
Descend the ascent route back across Horseshoe Basin and up to Sahale Glacier Camp, then out via Sahale Arm and Cascade Pass. An alternate approach via lower Horseshoe Basin exists but involves harder cliff bands and brush, and is not the more commonly used option.