The winter route to Boardman Lake involves several miles of road walking from the Mountain Loop Highway followed by a short stretch on the actual Boardman Lake Trail. Depending on the time of year and the snow level, you may be able to drive farther up the road.
From the Mountain Loop Highway start walking on Forest Road 4020 for 4.8 miles to the Boardman Lake Trailhead, and then follow the Boardman Lake Trail for just under a mile to the lake. Be sure to bring a map in order to stay on course at a couple forks in the road. With enough snow, the trail to Boardman Lake may be obscured, requiring some winter routefinding.