Ceramic tiles for outdoor use must be frost proof, able to withstand the stresses of freeze thaw conditions. When you are purchasing the tiles make sure to ask if they are suitable for outside use. Non frost proof tiles can delaminate with changing weather condition. If the tiles are on a walking area, paths, steps etc. be sure to choose unglazed or slip resistant surfaced tiles. Unglazed quarry type tile do not require sealers to finish the surface. Sealers can become extremely slippery and dangerous when wet.
Do not install outdoor ceramic tiles on any type of wooden material. When plywood gets damp it will expand and easily causing tiles to come off. Install tiles on concrete, masonry surfaces or special cement boards for exterior applications. Make sure the surface is clean and dry before applying the adhesive.
Ready made adhesives should not be used for ceramic tiles outdoors. Cement based acrylic thin set mortars will work best. When applying the bond material complete coverage is important. Any air voids between the tile and the surface can fill with water and potentially freeze causing a bond failure. Mortar should completely cover the surface, then completely cover the back of the tile ending up with about 1/8 of an inch of mortar between the two surfaces.