Start from the campground of Slim's River West
trail and walk southwest to the base of Observation Mountain until you reach Canada Creek. The creek is braided and will have to be crossed multiple times. If it is really deep, you may have to head to your left (east/downstream) until it is braided enough to cross. Cross all sections of the creek until you get close to the mountain (just under 3 kms from the campsite) and then walk upstream (west).
Continue following up Canada Creek until you reach another creek which flows in from your left. This is Columbia Creek. Turn left (south) to follow along the left (east) side of Columbia Creek. There should be rock cairns along the way to guide you up the creek. From where you started to follow Columbia Creek, it is 1.2 kms to the trailhead up the mountain (the Parks Canada website says it is 500-800 m to the trail, but this is wrong). If you don't have a GPS, an easy way to gauge where the trailhead is, is to look up Columbia Creek for a treed slope on the right which comes down to the creek bed. The trailhead is on the left, shortly after this treed slope. If you were to continue walking on the left side of Columbia Creek past this treed slope, you would soon hit a rocky cliff which comes right down to the creek bed and would force you to walk in the creek. The trailhead is just before that cliff.
The trail becomes steeper now as you climb up to the rocky ridge. The views of the valley below open up and are fantastic. Follow the ridge up to the vegetated mountainside where the trail climbs steadily up. Soon the trail will become faint, and will head to your right (west), cutting across the mountainside.
Cairns will mark the route, but if you lose the trail, just pick your way up to the top of the ridge (heading to your right as much as possible). On top of the ridge is an open plateau. You want to traverse this plateau (south) for 3 kms to the end where the Kaskawulsh Glacier lies beneath. There isn't much of a trail, so just walk across the open plateau towards the glacier (you should be able to see a bit of it by now). At the end of the plateau, rest and soak in the view. When you're done, follow the same way back so you don't end up over a cliff or bushwhacking for hours.
You can also climb to the top of Observation Mountain from the plateau if you feel like it (this will add time, distance and elevation gain). You can't see any more of the glacier, but you do get a view of the surrounding valleys and mountains.