Park below the dam, cross it and take forest roads around the west side of the reservoir. As you come out of the forest there is a fast descent. Continue past the turn off to Nant-llwyd which is worth taking if you need a short cut or you are on your second lap. Soon after you reach the Soar y Mynydd chapel.
Turn left, over the top to the head of the Doethie Valley itself.
Follow the fantastic singletrack beside the river for miles. It's mostly flowy, with one wetter area, and a couple of rocky steps.
When you eventually reach the road, take the track on the left, up the valley, back to the forest road.