There is plenty of roadside parking in the A5 laybys as well as a pay and display car park by Idwal Cottage Youth Hostel. Good place to start is at the Idwal Cottage. Go through an iron gate and follow the path towards the summit. Here's my Endomondo route (switched off too late, in a car).
LinesThe Banana Gully (1) is an obvious choice, when the snow cover is proper one can enter it from the summit. Another option is the line no 2.
This vid shows just a beginning of the banana gully line (1):
This one shows the second line (2):
The easiest approach is from the layby next to the Gwern Gof Uchaf camp. Cross A5 and head to Glan Dena. Now follow the path to the Ffynnon Lloer. When you get to the tarn you can choose a line depending on the snow conditions.
LinesFrom the Ffynnon Lloer you can traverse to the summit of Carnedd Dafydd. It's quite mellow ascent, so good for skinning. The descent can be started from either summit or the Cefn Ysgolion Duon saddle. Endomondo history for that trip is here.
The footage showing some parts of the descent.