Make sure you select a tile that is appropriate for outdoor use. To ensure a clean surface and proper bond, it is recommended that you clean the concrete with warm water and TSP (trisodium phosphate). Use a stiff bristle brush to really scrub in the solution and then rinse well with warm water. If installing tile on sealed concrete, it will be necessary to grind the surface down before starting so the thin-set will bond. Consider renting a scarifying machine.
If any steps have cracks all the way through them, they will need to be repaired before they are tiled. Lay a piece of crack isolation membrane over the area and cut it to size (Image 1). Trowel thin-set onto the tread and riser (Image 2). Set the membrane over the crack and press into place; smooth it out using a trowel (Image 3).
Dry-lay all your tiles ahead of time to plan the layout and prepare any special cuts that need to be made. Set up small stacks of tile ahead of time to allow you to work for longer stretches without having to fetch more tile.
Mix thin-set mortar according to directions. Use the flat side of trowel to apply the thin-set and the comb edge of the trowel to create grooves (Image 1). This will give you a really tight connection to your slab. Set the tiles by gently sliding into place. Use spacers to achieve consistent gaps between tiles (Image 2). When tiling the steps, start with the risers. Let the thin-set mortar cure overnight before adding the grout.
Remove tile spacers. Remove any excess mortar that sticks up between the tiles with a utility knife. Mix the grout according to manufacturer's directions. Mix only one bag at a time to keep color and thickness as consistent as possible. Dump out approximately one-third to one-half of the mixture onto the tiles. When tiling a large area, it is best to tile, grout and sponge in manageable sections. Use a hard rubber float to pack the joints, pulling the float diagonally across the tiles. When the tiles are grouted, begin to clean away excess grout. Use a large damp sponge to wipe across the tiles in a diagonal line to keep from pulling the grout out of the joints. Rinse the sponge repeatedly until the process is done.
Allow two weeks for the grout to fully cure before sealing with a high quality grout sealer. This will make the grout moisture resistant, which is especially important on outside tile jobs. The final step is to apply an exterior sealing caulk around the edges of the house to keep water from seeping into areas where it should not be.