Place the arch at the top of the window and mark both countour of the arch with a pencil.
![Step1](http://artmoulding.com/wp-content/uploads/2017/03/Step1.png)
Place the side trims at the sides of the window and mark the countor of the trims.
![Step2](http://artmoulding.com/wp-content/uploads/2017/03/Step2.png)
Take note of the intersection of the lines you draw for the arch and trims(black circle). Draw anoher line marking the intersection.
![Step3](http://artmoulding.com/wp-content/uploads/2017/03/Step3.png)
Place the arch on top of the window again and mark the join lines.
![Step4](http://artmoulding.com/wp-content/uploads/2017/03/Step4.png)
Cut the arch and the trims along the join lines that you marked previously and glue them to the wall.
![Step5](http://artmoulding.com/wp-content/uploads/2017/03/Step5.png)
Place the trims on the side of the window again and mark the join lines.
![Step6](http://artmoulding.com/wp-content/uploads/2017/03/Step6.png)