May i suggest you park at Cors Y Gedol, and then as you leave the top carpark, where there's enough parking for around six or seven cars, go through the gate just behind the parking area, which goes off to your right.
In about two hundred yards or so, you will see the ruined collapsed burial chamber off to your right.
Continue along the tarmac road for another half mile, down the hill and you will come to the famous Drovers Cattle shoeing station called lletty Loeger, which i think is now a holiday cottage or persons home, and Pont fadog bridge is just below, with the beautiful river flowing down to the coast.
Check out the 18C dedication stone on the bridge, which is dead centre on the right.
Just imagine the stories and number of cattle who have passed over this bridge, during the height of the Drovers trade in the early to late 18C
Head up the hill, and at the top of the hill through the gate, you will come to a junction, with the Drovers Scotch Pine trees looking weather beaten , behind the wall, off to your left.
You need the path that goes off to your right, because if you went straight on, you would eventually come to Pont Scethin, another legendary bridge in these parts of the Rhinogs.
Take the right turn and this follows the very desolate mountainside, eventually leading to the pass at the top, named Bwlch Yr Rhiwgyr, its always directly in sight, right at the top of the hillside.
Right at the top of the pass, when you have reached the Bwlch, in about two miles, you will see there is a pathway up to your left.
Follow the famous Great Wall of Wales, that will eventually bring you to the summit of Diffwys in around an hour or so.
If you keep this wall as a marker, you cannot go wrong, and its best to use the same route for your return, as doing a circular route, involves traversing a lot of boggy ground, some of which cannot be avoided.
This is one of my favorite walks, and one i do on a regular basis, as you virtually guaranteed total solitude and quiet, and the views from Diffwys, towards Cadair Idris and beyond, are mind blowing on a clear day.