The ground in this area can be very boggy so good boots, waterproofs and warm clothes are essential. It's about 11 miles and will take five hours.
Start in the village of Esteddfa Gurig (Sat Nav SY23 3LE) at grid reference SN796840. Enter the farmyard. A metal gate marked "All Walks" at the time of writing takes you to a stream. Follow this along a path initially west, then turning north up to some derelict mine workings.
Before reaching them, the path turns left at a waymark and becomes a sheep track.
Once you get close cairns guide you to the summit of Pumlumon Fawr. Cross the fence and a stone refuge and trig point populate the summit.
Head for the next cairn and then follow the line of the fence that runs generally east to spot heights of 691m and 727m. Leave the fence where it turns the corner at a cairn at the 727m spot height.
Head generally north now, towards some crags, keeping to the high ground until you cross a gulley. The northern tip of Llyn Llygad Rheidol reservoir is your objective, generally following the north side of the gully.
Reaching the reservoir, you cross the outflow. This should not present any difficulty - walk across the concrete strip where the lake overflows into the stream. Once on the other side, follow the access road.
The road contours the mountain and emerges onto a road by a reservoir. Turn left, past a pumping station then to a crossroads.
Continue left and downhill to a farm, cross the cattle grid and up the incline past the farm, onto a track heading into the woods.
Go through a gate, head towards the woods and a stream, turn right at the stream and follow it for 300m before entering the wood at a waymark. This track will emerge on the east side of the woods.
You will emerge at a fence running north-south. Cross the fence and head down the hill past the old mine to the start point.