Follow the tourist path from Pont Y Daf carpark, and dress accordingly for the conditions.
Do not forget that Pen Y Fan is nearly 3000ft in height, and when its chilly by the roadside of the A470, it will be well below freezing at the summit, and possibly very windy as well.
When living in South Wales, i climbed Pen Y Fan so many times, that ive lost count, it must be well into the hundreds, and it was rarely calm on the summit, even in Summer.
Its the windiest location ive ever experienced in the near forty years ive been a walker.
Go fully prepared for such a high exposed mountain in winter, and you should enjoy a great walk in the hills.