If you've got a plain wooden window sill in a bathroom or kitchen and you want to dress it up, the standard approach is to simply add some decorative wood trim. A more interesting approach, and one that can look attractive in some rooms, is to tile under it, where you would normally put that trim. Bullnose tiles are finished and rounded along one edge, allowing you to make a border out of the tiles that can tie the window into the other tile colours and patterns of the room.
Dull the wall under the window sill, along its length, with sandpaper, sanding it by hand for as far down as the height of your tiles. So if the tiles are 5 inches deep, sand about 5 inches down from the sill along its length.
Measure the length of the sill. Find and mark the middle with a pencil.
Spread tile adhesive over the back of a bullnose tile, using a notched trowel.
Press the tile to the wall under the sill, on one side of the centre mark. Set it so the unfinished side is right under the sill and the finished edge is facing down.
Hang the remaining tiles along the edge in the same way, working from the centre out in both directions toward the ends. The self-spacing nubs on the sides of the tiles should keep them separated.
Hang as many tiles as will fit along the bottom of the sill. Stop when the last tile on either end is within one tile width from the end of the sill. Let the adhesive dry for 12 hours.
Put masking tape all around the tile to protect the wall from the grout.
Apply grout the tiles with a grout float, scraping it over the face of the tiles, pressing the grout into the spaces between. Take up the excess grout with a dampened sponge. Let it set for 24 hours.