Head for Caernarfon. 

If you are coming from Conwy direction then you will enter Caernarfon on the A487. Pass Morrisons on the right hand side and go straight over the roundabout. You will find yourself on a flyover road which will veer to the left with "The Eagles" pub on your right. Continue straight on that road after the left bend and head out towards Beddgelert (A4085). You will eventually come to a roundabout in the village of Caeathro with a Shell garage on your right hand side. Go straight over the roundabout and head towards Waunfawr. Once in Waunfawr, continue straight on past a chapel on your right, past the village shop and a motor garage on your right. You will eventually come to a small church on your right hand side. STOP! Bryn Derwen is opposite on the left. Waunfawr is approximately 4 miles from Caernarfon so it won't take you long.
 

I suggest you park outside the house on the road when you arrive. I will show you where the off road parking is.  

If you're using the What Three Words app.... its:  

///woodstove.verbs.sqeezed 

This will place you outside the front gate of the house.