You can access it from Loch Duich/Glen Shiel, at the foot of the descent I think you’re referring to when you say ‘best singletrack in the UK’.
You can stay at the remote youth hostel (Allt Beithe) in Glen Affric, climb up to the Gates of Affric (Bealach an Sgairne) descend towards Loch Duich but take a right turn up Bealach na Sroine which leads to the Falls. You could do the reverse of Scotroute’s suggestion and continue from the base of the falls past the Iron Lodge, past Maol Buidhe bothy, on towards Bendronaig lodge before finally descending to Attadale train station. You would need to work out some fancy transport logistics to get back to your start point though!