From the boat ramp at Convict Lake, take the trail to the left that contours the east side of the lake. Stay on the trail longer than you think you should, until you enter the thick aspens at the west end of the lake. When you find yourself walking on a little causeway, delineated by rows of rocks on both sides, look for a use trail to the left. The trail parallels a line of slide alder, and makes an end-around to avoid a bushwhack. If you find yourself fighting the brush, backtrack and find the trail again. It is much easier and more pleasant to stay on this use trail.
Once through the slide alders, climb talus toward a saddle and then when they come into sight, choose one of the steep, loose ribs that lead to the Northwest Ridge. It is in your best interest to avoid the gullys between the ribs, they are loose, loose, loose, and ready to roll. When you finally make the ridge, do your best to stay on the crest , and enjoy the plentiful exposure. The rock is extremely loose, and large amounts of rockfall will be generated. If you are a trundler at heart, you will love this route. Continue up the ridge to the summit. You may wish to make a round robin of the trip and descend the easier (and less loose) East Slope route.
Normally you can just bring your typical dayhike gear, but early season may find you wishing for an axe and crampons. There is no water after you leave the lake shore, unless it's a warm spring day and the snow is melting. Plan accordingly.
A little history...
Mt. Morrison is sometimes called the "Eiger of the Sierra", notorious for its loose rock, especially on the North Buttress. But all of the routes have some loose rock, and the Northwest Ridge has more than its fair share. The Northwest Ridge was first climbed by Norman Clyde (who seemingly had 1st ascents on about 90% of the Sierra peaks) in 1928. This route, unlike the East Slope, is not often attempted. As a note of interest, Glen Plake, (in)famous extreme skier, has many winter and spring ski descents of the peak.
More info, courtesy of Diggler. It was interesting how Clyde decided to climb Morrison - John Mendenhall (another prolific early Sierra hardman), after staring up at Morrison so often while on camping trips with his parents at Convict Lake, asked Clyde (probably the most knowledgeable source around) if Mt. Morrison had seen a first ascent (John being interested himself, I'd suspect)- Clyde subsequently hopped in his car, drove to Convict Lake, and made the peak's first ascent. Mendenhall said he kept his plans secret after that.