There are two main routes up Ben Ledi. By far the more popular one starts from a parking area on the west side of the bridge at GR NN586092 (turn left off the A84 about 1¾miles beyond its junction with the A821 at Kilmhog). Look out for cyclists and pedestrians on the Rob Roy Way as you're parking. This is the route shown on Walkhighlands. It's fairly unrelenting, and I found the bit through the trees at the start rather dark, dank and uninviting - basically just something to get over and done before emerging on to the open hill above. AFAICT from the OS map there is no made path for the first 2km or so of the Walkhighlands route down from the summit, across the moorland before you reach the top of Stank Glen, although you could just retrace your steps the way you came up.
A second option is to start from Brig O'Turk (if you can park in the village without causing an obstruction, otherwise in the car park at GR NN546065) and head more or less due north through Glen Finglas on a good landrover track, then NE up Gleann Casaig until (approximate GR NN544100) you are roughly due west of the summit of Ben Ledi, which is just over a mile away across open moorland: my recollection is that there is no made path but the trodden paths are easy to follow through the heather, and there's no particularly difficult (bouldery/craggy) ground that cannot easily be avoided. This is longer than the first route, about 10 miles in total, out and back, but the gradients are generally easier because of that. IMO the Glen Finglas approach is nicer, with views over the reservoir. It's a longer outing but that makes the beer* at the Lade Inn afterwards all the more welcome! OTOH if you prefer made paths to landrover tracks then maybe the route from the East edges it.
* And an appropriate non-alcoholic beverage for the designated driver.