You have a choice of routes! ...
Just as you leave the village heading towards Dolgellau, there is a layby on the left. Park here then walk about 50 metres in the direction you were driving. You will see a sign posted footpath which takes you up the mountain on a steady gradient. Stay on the path until you reach the old mill at the actual mine site - about 30 minutes walk. Continue walking passing the mill ruins on your right towards the obvious lined tunnel (water shin deep) This will take you into the open quarry. Look to your left and you will see the adit. The water here can be over waist deep, but during drier periods, knee deep. If the water is deep be vry careful that you don't walk into the submerged wagon!
There are about 5 big chambers, winch bases, the remains of a big crane, boiler that powered the crane and the porcelain pots that acted as a chimney.
A small place, with a bit to see, worth a visit.
There are some pictures on this very site.