I am building a raised hearth for my woodstove, and am ready to install the manufactured stone veneer. I am familiar with doing this with the small stones on walls, but unclear on the process for the hearthstones. They are 19x20 inches. The surface of the hearth is cement board. Do I use a notched trowel, just butter the back of each stone and set it in place, do both, or something else? Thanks.