From the intersection of Lake Cavanaugh Road (from Highway 9 southbound) and Granstrom Road (from Highway 9 northbound), follow Lake Cavanaugh road another 5.2 miles to a logging road turning uphill to the left (Elev. 918' N48 20.628 W122 03.692). Park in such a way as to not block the gate, or better yet, park on the other side of the road. Do not enter with your vehicle if the gate is open - this gate may be locked at any time. Start biking up this road, which after the first hill will be mostly flat. Stay on the main road, and ignore branches coming in from the left at 1.0 and 1.5 miles. You will generally be paralleling Pilchuck Creek heading east. At 5.5 miles, take the left fork, which will start uphill (Elev. 1350' N48 21.930 W121 57.508). In a quarter mile, turn right. Follow this fork 2.0 miles to an intersection with roads coming in from both sides of Day Lake (N48 23.065 W121 56.276). As you approach this intersection, you should have a view of your objective that looks something like the photo below.
At the intersection, stay on the right fork, which will quickly head uphill on Big Deer's west flank. Continue until on the south side of the mountain at an elevation of 2236' (N48 22.161 W121 55.348). Total distance from the gate at this point should be about 9.0 miles, and now its time to lock up the bikes and start walking. Find an old roadbed which starts a slow climb in an east direction on Big Deer's south flank (this road is identified on the USGS map with a dashed line). This road will eventually switchback up the peak to about 3600' on the south side - expect it to be quite brushy in places. Once the road fades out, scramble straight up to the 4240' false summit on the south ridge. From here, you have a great view of the scramble route to the summit.
Take your time here to study the route before starting up. There are a number of variations possible, and it should be possible to finish the climb at class 3.
Once on the top, you will find the views are pretty good for a 4349' peak.