Assuming you're coming from the top of Mt. Wilson, look for this trail to re-appear off the left side of Mt. Wilson Toll Road
. Eventually, you'll come to an intersection with a few trails to choose from. Stay right at this intersection (do not take the trail that has a TRAIL sign as that is the wrong trail) and keep going down.
There may be a few washed out sections, so it might be best to walk. You'll come to a sign that says "First Water", keep going straight here. Watch your speed as you start getting closer to the bottom, as you'll start to see hikers and the trail can get busy on a nice weekend day.
Keep heading down and you'll eventually come to a road. You'll see a trail on the right side of Mt. Wilson Trail, so hop on that and you'll shortly be at the bottom. Now you just have to wait for your ride as you probably beat the driver down!