We just installed the pennies (we had glued to the mosaic tile mesh backing) directly over the existing ceramic tile in the vestibule (foyer/entryway) with regular thin set. We then grouted it with laticrete epoxy grout in chocolate truffle. It was really very easy and i love the way it came out.. most people that first see it don't realize that it's pennies. It took 3,889 pennies (it's about 3'-8" x 5'-8")...so $38.89 for app. 24 sf of tile is not bad at all.