Place the arch at the top of the window and mark both countour of the arch with a pencil.
Place the side trims at the sides of the window and mark the countor of the trims.
Take note of the intersection of the lines you draw for the arch and trims(black circle). Draw anoher line marking the intersection.
Place the arch on top of the window again and mark the join lines.
Cut the arch and the trims along the join lines that you marked previously and glue them to the wall.
Place the trims on the side of the window again and mark the join lines.