I'll add a couple of things - the Manker Flats trailhead is on the left-hand side of the road. Just below the trailhead the road splits, and there's a island (lots of trees in the island) between the uphill and downhill lanes. I've always found it easier to go slightly past the trailhead itself where the island ends, make a U-turn, and park headed downhill. You can park on either side of the road. The TH itself isn't terribly obvious, but usually there's a portapotty there. And it can get VERY busy on weekends, so getting there early (6-7AM) is a good idea.
