Taking this one step further: once you have folded the piece, finger
press the fold; then open it out again, carefully, and *sew* along the
fold (keeping it in place). Trim off the excess and you have it!

I love attic window, too, for all the different illusions you can create
with it, simply by colour choice and placement of pieces.
..
In article , Muggywort
writes
snipped

BTW, I read somewhere another way to do them...after stitching the
attic pieces to the side and bottom, fold one attic piece at a 45
degree angle over the other and then do a ladder stitch to avoid
setting in the angle. That's what I usually do, but it does make for
extra hand work.

Muggy
--
Best Regards
pat on the hill