The Old South Fork Trail doesn't show on all maps, but it is very visible and well travelled.
There are two ways to connect with the trail. The first is to jump off the trail at the stream crossing from Long Lake I have done this a couple times, but particularly don't like it because of the obstacles encountered down to South Lake.
Since I normally descend Army, I cross over at the end of lake 1 and junction with the trail from New Army. From there I just walk off the gentle slope and head south thru the trees and after a half mile or so meet up with the new trail to South Lake.
When you are at South Lake, turn to your left and you'll see a well used path that follows the stream down the canyon.
This trail, meets up with the main trail just before the first stream crossing (if hiking north). At that point, it's 3o minutes or so to the trailhead.
FYI..... you will come to an old trappers cabin on the trail This cabin is about quarter of a mile from the main trail junction.
paul