Starting with the bottom step, carefully set aside the stones, making sure to keep them in the right order. Use a drill with a paddle bit to mix mortar to the manufacturer's specifications; it should be the consistency of brownie mix. Spread a layer of mortar along the front edge of the step as a base, then set the first layer of stones on top of the mortar, spreading mortar in between each stone. Use mortar to set the second layer of stones on top of the first. Check the stone facing with a level, adding or removing mortar as needed to level the structure. Use a wet sponge to wipe away excess mortar from the front of the facing. Repeat for the other facings.