Start at Bunny Flat Trailhead (~6900')
blast up to Horse Camp (1.7mi)
take the standard "Avalanche Gulch" route (up and around/through the Red Banks, up Misery Hill to the summit plateau, then up the final summit block)
Make summit on Mt. Shasta at 14,180'
back down the summit block and across the summit plateau and dive off towards Shastina
carefully route find above, through, under any crevasses in the top end of the Whitney Glacier to gain the ridge that drops you directly to the saddle between Shasta and Shastina
climb to the Shastina summit.
drop back down to the saddle and down the snowfield or scree to find the fairly well traveled trail for the "West Gully Route" to take you back down to Horse Camp
continue back down to the Bunny Flat trailhead.