Using two parts of a length of wallpaper for the ceiling slope

There are two methods to do so, one is to go behind the angle of the vaulted ceiling plan, and the other one is to place the peak ceiling under the actual ceiling.

In case you want your window to follow the slope of the ceiling, it would be better to favor a trapeze window that will perfectly fit the slope.

A piece of furniture in the shape of stairs is another option to easily create optimal storage space, especially since it joins with the slope of the ceiling