Normally, you'll trim out a window interior with wood trim. But that's not the only way to do it. For a unique look that's also durable and easy to clean, consider tiling around the window. Bullnose tiles are made to provide exactly the kind of finished edge you need, with one side of the tiles bevelled so they can sit uncovered against the wall. Two-sided bullnose, in which two perpendicular edges of the tile are bevelled, will dress out the corners.
Mark lines out from each side of the window, extending line of the window out past its edge, using your level and pencil. Extend it by the width of a tile. Mark it in both directions, so you're left with four drawn corners on the wall, sitting diagonally off the four corners of the window.
Spread thinset mortar on the back of a two-sided bullnose tile, using your putty knife. Press the tile to the wall, within one of the corners you drew off a corner of the window. Set it so the two finished edges face outward. Repeat at each corner.
Measure between the inner edges of two corner tiles. Find and mark the middle of the span. Mortar and set two one-sided bullnose tiles on either side of that mark, setting them so the finished sides face outward. Put spacers between them.
Set the rest of the tiles along the edge, building outward from the middle, setting them with the finished edges all facing outward, and putting spacers between them. Use a tile cutter to cut the tiles at the ends, putting the cut side next to the corner pieces. Repeat on each edge of the window opening.
Allow the tiles to set for a day. Remove the spacers.
Spread grout over the tiles with a grout float, forcing it into the spaces between the tiles. Wipe off the excess grout using a damp sponge.
Wear goggles when cutting tiles.