Take the first minor road west after Ardallie, and head in a north westerly direction. The first farm on the eastern side is Humblecairn.
Sadly this cairn will soon vanish probably to be ploughed out. What remains is a 13 meter wide grass, sometimes cow, covered mound standing at 0.2 meters high. Cairn material scatters the ground but the bigger stones mentioned by Canmore have long gone.
At least, on this occasion, I made it before it was totally destroyed.