Exit Dolgellau by road towards Cadar (or Cadair) Idris and you will come to a large car park with overflow (costing £4 in 2012). You can either walk the well trodden and easy to follow pony path and come back down the same way. Or descend by the Fox’s path which is a very dangerous slope of scree path starting just beyond the summit. An information board in the car park lets you know that it is 734m of ascent to the top of Cadar Idris (893m). If you want to ascend further then you could try the Minffordd Path. I haven’t done the Minffordd Path walk so can’t vouch for how easy it is to follow the path.
I would recommend ascending rather than descending via the Fox’s path. Even that is quite dangerous!
We walked the route shown here on 20 August 2012 and unfortunately for us the tops of the mountain range were under the cloud. On a good day the views are spectacular.