As Colin says, once you have crossed the River Gairn on the A93, look for a side road on the right that leads down to the stables, it is opposite the post box, go down the side road for a very short distance, maybe 15m and you will see a footpath to your left which passes around the back of the house, follow this pleasant trail, a rail line that was constructed but never completed, into Ballater. After the car park just keep going straight ahead and you will end up in the centre of town.

Thanks Colin & Brian, most helpful.