A splendid day in early January, cold and clear. Our route doesn't seem to be described by others, so here's the detail, shown on apps using Open Street Map as a base.

We walked along the 3 Lochs Way from Arrochar (initally N to cut onto it), then followed it until it crossed a wide track going uphill. We followed this nearly to the end then picked up the path going directly uphill from near Arrochar station to the summit. Amazing views. We also went a short distance east from the trig point for a better view of the loch.
We then cut south west down across the heather to join a soon-visible path. Just before getting to this path, which was on a flattish area, we joined a steep grassy path descending, this may or may not have linked to the higher paths without the need for heather bashing. Nothing shown on the map anyway. Our path then led gently downhill, through woods (bit of GPS checking needed here or guess depending on where the bits of wood help to cross the bog) then along by the pylon line. We joined our outward route, followed it back and zigzagged down to the main Arrochar street for an initially unsuccessful search for a tea shop. We ended up enjoying drinks and cakes in the porch of the Olive Branch next to the garage. This excellent round extended the route into a short day rather than a half day walk up and down from the station.