Check the concrete stairs with a 3-foot level. The level will help you see any large dips that may need to be filled in with a mortar bed before installing the tile. If the concrete stairs usually have water puddled on them after a rain, you will need to add a mortar bed on top before tiling. A mortar bed is made with the same thinset that you use to install tile; it is mixed with less water and laid in a thicker layer on top of the existing concrete. The mortar bed, if needed, needs to have an even surface for the tile and the correct angle to prevent water from puddling on the tile stairs. Allow the mortar bed to thoroughly dry before installing tile.
Buy the right type of tile for outdoor use; it will have a slightly sandy surface to prevent slips and falls when wet. You will also need to buy thinset mortar, grout and grout sealer. Other supplies needed are a tile cutter that you can rent, a trowel with the appropriate notch depth, a float to apply grout with, a five-gallon bucket to mix the thinset and grout, spacers, sponges and gloves.
Clean the concrete stairs first. Use a broom and a hose to clean off any specks of dirt. Allow the stairs to dry thoroughly. Apply a mortar bed if necessary and allow it to dry before installing the tile.
Mix the thinset according to directions; thinset is the mortar that is used to secure the tile to the subsurface. Latex additive is sometimes used in thinset, check with the manufacturer for your application. Use the notched trowel to apply the thinset to the concrete stairs. Apply the thinset to small sections, no larger than 3 x 3 at a time.
Set the tile that doesn't need to be cut first. Gently push into the thinset and apply a slightly twisting motion until the tile is set straight. You may need to back butter the tile before setting; do this by using the trowel to apply thinset to the back of the tile as well. Place the plastic spacers in the upper corners of the tile to keep the tiles straight and give a good line for the grout.
Wipe off any thinset that has splattered onto the tile; make sure you do this while it is still wet or it will be too difficult to remove when dry. Clean the thinset off the trowel and clean the bucket before any of it dries; it can be removed with water. Remove the spacers after the thinset has set up, about 2 hours.
Mix enough grout to do the whole job if it is less than 10 stairs. This will ensure that there will be no color variation by mixing more than one batch; even white grout can turn out in different shades. If the job is larger than 10 stairs, carefully make a note of exactly how much grout, water or latex additive you've used so the next batch is the same.
Apply the grout to the tile with the float; the float has a flat rubber surface. Spread the grout into the grout lines using angled strokes; don't worry about getting grout onto the tile surface. Apply the grout from top to bottom, then right to left. Wipe off any excess grout from the tile.
Allow the grout to dry a few hours. Use a damp sponge to remove the excess grout from the tile and allow to dry again. When the grout has dried, there will be a film on the tile. Use a clean cloth to buff the tiles and remove the film. Done.