The trailhead is at the corner of Russell Road and Foresthill but Google Maps puts it at Canyon Dr. and Russell Road, park at the hairpin turn just before you get to Canyon Dr. It's just a couple of minutes from the Starbucks just north of Russell. If you know what your are looking for in the satellite image you can see the old stagecoach road making its way down the mountain to the American River confluence.
One of the reasons I like this trail is that there are a couple of benches at key stops to sit and view the American River Canyon.
The Upper Stagecoach Trail, heading north, takes a hard right turn down the mountain to get back to the old Stagecoach Trail. This turn is not well indicated. We kept walking straight and missed the turn to spite the fact that there are nice tail signes posted, they just didn't say "--> to Stagecoach Trail". The trail we eventually took back is the one behind Sharon, it looks like a gully for dirt bikes more than an hiking trail. None the less, the Flood Road Trail was nice walk on a trail less traveled, not much of a grade either.