If you know a bit of Photoshop, one way would be to start by Googling {seamless tiling} or {seamless pattern tutorial}. These will show you how to get a nice repetitive pattern that you could use for the background.
The tbe next step would to figure out how to make the four fold symmetric decorative "window". The appropriate tools for that would be the flip horizontal & "flip vertical" commands.
Unfortunately, I have to leave for the day, so I can't put a full tutorial together for u, but this should get u started at least until I return tonight or someone else comes along and can fill in the details.
Cheers,
Tom M