This s one of mine.
http://www.quiltingboard.com/picture...s-t207659.html
You need a bold contrast for the sills to get the depth. While i like to use a wood look fabric, any tone on tone or mottled fabric would look fine. You want the contrast tho. Colors would be fine...experiment and have fun with the process.
Shadow box is a great pattern.. Notice the stark contrast used to get that "shadow". You should buy that pattern if you want to tackle it.
Attic windows are simple to make once you get the hang of them. You can miter the corners or use a half-square triangle technique to mimic the mitered corner in the attic window.
The size of my sills are no larger than 25% of the block itself.
Sandy