There are many ways to do this and I have listed what I think is the easiest and best way.
1. First draw your wall covering so that it is the height of the wall and lines up with one side of the window like shown below.
2. Go all the way into your wall covering until all is blue, Once blue draw a line where the top left corner meets the timber covering. Check below.
3. Now repeat step 2 for the bottom left corner of the window.
4. Push/Pull both wall coverings over to the right side of the wall. It should look like below.
5. Now do the same as steps 2 and 3 but for the bottom right hand corner of the window.
6. Once done now just Push/Pull the wall up to the other wall covering. It will now look like below.