Head up Fire Road 437 as if you were going to the South Boundary Trail at Garcia Pass. About a mile before the South Boundary Trail trailhead, there will be a jeep trail intersection on the right; it splits into two trails - take the trail to the right. It is not marked, so use the MTB Project mobile app
to locate the trailhead.
After about a mile of easy pedaling west, you'll reach an intersection of several roads and trails, including #164; look for a doubletrack trail on the right that heads downhill (see the video). After around two miles, be on the lookout for a trail that splits to the left (you'll be going pretty fast, so pay attention!); if you reach FR437, you passed it. At this point you'll climb for a bit... now, this is where I might have gotten off track and missed a turn-off to the right, so watch carefully... if not, then the wide trail will end at a steep goat trail that drops into a ravine. This is where the trail disappeared for me, and where I began my hike in the woods, which eventually led me to another road bed.
Once I found the trail again, it was easy to follow after this, as it enters and descends Mondragon Canyon. The last mile or so is fast and rough. When you reach the creek at the end, look for a log crossing to the left, ending at Hwy 64.