its hard to explain,if you just go right to the top on the fire roads you should see the track,it crosses the fire road like 4 or 5 times

 go to furnace( waterfall on side of road), turn left if your comin from north wales side just after the waterfall up a tiny little road, follow the road up for lik mile or 2, then turn right at the top, up the forest road another mile and you'll see it cross the road as he said ^^