1) Rubber Cement (available at Home Depot, a good product is Elmer´s).
Apply at least 2 heavy coats to the back surface of each tile. Do not leave tiles in direct sunlight, as this will dry the rubber cement and it will be very hard to remove later.
DO NOT use tape. Using tape will make grouting more difficult, exposing tiles to damage.
2) Thin-Set Mortar (or any comparable high-grade thin set mortar, like Laticrete 254 Platinum Multipurpose). Mask off tile to eliminate grate in all relief pieces and to avoid scratching finish. Remove excess grout by using water only. All grout must be removed from the surface before it dries to eliminate removing the lacquer coat and scratching metal finish. Acid or other chemicals used to remove excess mortar may destroy the protective coat and change the color of the metal. Do not use harsh chemicals on metal tiles.