SN7768924517 From the Youth Hostel cross the road and go south past the churchyard on a minor metalled road. This descends steeply to cross the Afon Sawdde, then climbs to a corner near Gellygron farm. Leave the tarmac here and continue on a stony farm track. This climbs steadily to reach the fenced edge of the open pasture. Having entered access land you're now free to venture off path at will (though there's not much to be gained from doing so).

SN7808023204 The path is vague at first, but keep heading roughly south and it soon becomes clear. A long gradual ascent brings you to some old quarry workings on Carreg yr Ogof. It's worth branching right off the main path here to wander among the layers of limestone outcrops, which offer some very modest micro-bouldering. Ogof means cave, and a good one can be found if you spend a bit of time hunting to the north of the summit (torch required, but no techy caving gear). Craig yr Ogof's trig point is perched on the edge of a tiny crag, and serves as a great viewpoint over the rolling farmland below.

SN7779121461 Continue roughly south past more outcrops, and areas of shattered limestone pavement, into a boggy saddle. Conspicuous shake holes here are evidence of more caves. The ground gets rougher on the final ascent to Garreg Las, and there's a choice of several trails among the rocks. The summit is crowned with two enormous cairns, thought to be of Bronze Age origin.

SN7774420329 With time you could loop west from here to take in neighbouring Garreg Lwyd, or head back northeast to visit Fan Brycheiniog. But for a shorter day simply retrace your path to Llanddeusant.