Approach from Boston Basin as described for all other routes on the mountain. I have something special to say about Sharkfin Col however. While approaching the climb we met a fellow with some knowledge that made dealing with this unpleasant obstacle much easier. (thanks again "Nelly"!)
The col is at the low point in the ridge between Forbidden and Boston Peaks. Selected Climbs in the Cascades recommends leaving the Quien Sabe Glacier at it's western edge (elevation 7,500 feet). Then, as normally described, you would climb some steep, loose rock (often rated 5.7), to enter a gully of loose 3rd class that brings you to the col.
But instead, we easily avoided technical terrain by climbing a hidden 35-40 degree snow couloir immediately on the right, not visible from the usual overview-of-Boston-Basin pictures. Climb this colouir for 200 feet to reach a notch in the ridge just 20-30 feet higher than Sharkfin Col. There is a decent rappel anchor here with fixed slings. We had a single 30-meter rope, and one rappel got us to a flat rock at the edge of the Boston Glacier.
Our climb was in August, and the snow couloir was full of snow, after what was I believe an average winter (2001/2002).
Once on the Boston Glacier, descend 200 feet to get around a protrusion of rock coming from the Forbidden-Boston ridge. Then, follow the glacier as crevasses allow to a broad snow entry onto the North Ridge.
Bring a 30 meter rope, possibly a half-rope that you can double for the rock ridge. We only found one pitch that needed a fixed belay.
Bring crampons, ice ax, glacier gear like prussik cords and a picket.
Bring a light rack of nuts and cams for the rock climbing.