Instructables

Step 6: Windmilled Four-Patch

Let's jump ahead to what the four patches will look like:

I put together two pairs of squares, each of which will have the seam allowance pressed to one side. Then I line them up against each other, so the seam allowances go in opposite directions, and sew them together. After that, I pop apart the ends of the first two seams, so that I can spread the seams at the intersection flat.

Notice how the pressing has rotational symmetry. This is important. It's also important that they all spin in the same direction.  Getting them to all spin in the same direction is managed by always having the upper seam allowance pointed up, as they go through the sewing machine.