Single small windows in odd shapes and sizes, placed in unusual locations, can be part of the charm of an older house or apartment. But these don't always lend themselves to easy decorative treatment.
Perhaps its an off-center dormer window, a small high window on either side of a fireplace, or a round or hexagonal shape over a stair landing area that poses a problem. By giving any of these a border of pretty tiles, they can be made to seem at least twice their size without sacrificing light or air.
For best results, plan the size and replacement of the tiles before fixing them to the wall. Use a paper tiles in various sizes to "try on" for size. Work on the floor until the placement seems satisfactory, then measures for the tiles and their placement.
A round or octagonal window can always be set in a square frame of tiles, and if the window has no sill the tile border can be used on all four sides. Fixing the tile to the wall will depend on the type of tile you're using, whether ceramic, vinyl, adhesive-backed, or any other. Be sure to ask for directions from the source that provides the tile. And when in doubt, consult any one of a number of reliable information booklets devoted just to tile products.