I've seen two fake cathedral windows on here. The simplest was just cutting a circle of backing fabric, drawing a square inside the circle, and placing a square inside that circle (with or without batting), and sewing along the line for the square, so that adjacent circles become squares, and the rest of the circle covers up part of the square. Then stitch down the folded over circle part over the square. (this gives a ragged edge), Or you can sew two circles together, turn through a hole near an edge of the inside fabric. Draw a square on the inside fabric, Sew two adjacent circles together, fold the edge of the circle onto the inside fabric, and stitch down. This has also been called a false double wedding ring (use striped fabric for the arcs for the rings).
The second way, which looked just like cathedral windows, at first I though it was a 10 minute block with a large square on the inside, lined with another square, and the edges of the square formed by the 10 minute block folded over the added square. But it was actually just 4 squares each with a square folded into a triangle sewn together on the sides that had a triangle edge. (is this the 5 minute block?) Then another square put inside the square that's formed by the folded triangles, and sewn down. But this quilt would need a backing (and batting if desired).