From the Bear Lake parking area (there will be great signage and there should be a established skin track but one still needs to pay attention), skin-up and head out on the Bear Lake trail. You'll climb above Bear Lake (amazing views of Glacier Gorge and Lumpy Ridge) looking for the Flattop Mountain trail.
Take the Flattop trail west (left) and continue climbing through the trees. The next trail junction you'll come to will be the Bear Lake-Fern Lake trail, continue on the Flattop trail which starts to climb the east ridge of Flattop Mt. To reach the summit of Flattop continue climbing for the next few miles along the ridge line. Depending on snow conditions there might be some boot packing higher up on the ridge
This is the approach for a few great alpine descents. One can also break off part way up to access Flattop's great East Face, offering great moderate open slopes.