Place the arch at the top of the window and mark both contour of the arch with a pencil.
Place the side trims at the sides of the window and mark the countor of the trims.
Mark the intersection of the lines that were drawn before for the arch and trims.
Place the arch on top of the window again and mark the join lines.
Place the trims on the side 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.