You can also catch the trail at Clear Lake Resort, or go a couple miles further down where the trail crosses the highway. The shoulder's wide enough to park on the highway. It's the same trail. Trail forks at a bridge. Left fork is more technical, lots of lava.
Trails rejoin in a mile or so, below the lake.
First few miles is rocky and technical, after that it's pretty smooth, through lush forest. Great views of the river, some beautiful waterfalls. Almost entirely downhill. At one point the trail goes through a little campground and the trail gets hard to find. But if you just keep riding in the general direction you will be able to find it again.