Here are some tips from quilters that have made several of these quilts.
 1) One of the tips I'd like to pass onto you is once I've decided upon the basic fabric I want to use,
I then take a piece of cardboard and cut a square out of it the size I think I want my window to be.  I use this all over the fabric looking for just the right place I want to do my cut-out.  Sometimes I need to re-cut my cardboard square cut out several times until I get the size that looks best, just saying square, but actually often prefer to use rectangle, its just my personal choice, vs the look I like.  I really do not like square wall quilts, so always look to make it rectangle. 
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3)  After I've decided on the size, I always make a sample block.  Cut out one block only and again do some serious thinking vs the size of ledge I'm going to use after I've determined the 2 fabrics for the ledge.

 Again, sometimes it takes several tries vs the size of the ledge and perhaps the fabric I chose for the ledge to give it just the right look/depth.  Sometimes I put the darkest fabric on the bottom and sometimes I put it on the side. 

4)  I try the fabrics out before I cut my sample block, where it is best to put the dark fabric. 

I think the important part of this is to make sure the other ledge is much lighter than the dark fabric, it doesn't have to be a light but its better to be on the lighter side so you'll get your dimension. 

The other option that is kind of nice is if the window fabric has lots of black background to it, then you can use two different solid medium different colors for the ledges... same degree of medium color. The colors not the contrast will give you the 3 -  dimensional look. You'll get lots of dimension with the real dark window.

Main Quilt Frenzy
Web Page
Page 1 

Instructions for sewing the miters.

Page 2   (here)
Some directions for fabric choices and making the squares 3 -  dimensional here.
Page 3
Real pictures
of Attic Windows quilts for inspiration.
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