Rhodiad-y-Brenin is on the B4583, pass through the village and take the minor road on the left, after 250 metres it bends sharply to the left. Park somewhere here on the verge but dont obstruct the bend. Once on foot take the small tarmac road to the right. Continue along it for 50 metres before turning right along Feidr Dwr (Water Lane) which really is wet and muddy. On reaching the next tarmac road, turn left and continue to Treleidr Farm, turn right in front of the hay barn (it's signposted) and follow the narrow path between hedgerows. (This can be wet too). Continue over the stile and bear left. A good detour here is to climb Penberry, it's on your right and best approached from behind, the view is stunning - you can see your car from here!
Once back down on the main path continue to the next stile and follow the coastal path westwards for about 2km. This is where you should keep an eye out for the rare chough -- they're black with red legs and make a strange noise. Also there are seals along here in the coves.
After you pass Porth-y-Dwfr, take the signposted track inland above the cove of Porth-gwyn and climb steeply (actually it's really very long and steep) between Carn Treliwyd and Carn Ffald. If you haven't collapsed at the top, continue down the other side to the settlement of Treleddyd Fawr. Walk through the farmyard, turn right and walk down the tarmac lane. Turn left at the lane and follow it round the edge of the settlement. Bear right onto the footpath past Ty Uchaf (a house with the name outside), cross the next field, turn right down the track to Trelewyd Farm and continue on to the road. Turn left on the road and follow back to your car.
I did this path earlier in the morning before most people were out and about and, for the first two-thirds of it, saw nobody - it was beautiful and peaceful. I'd recommend you take water and supplies as it is a bit strenous in places, but the views make it worth the effort.