I've had trouble in the past with my square in a square blocks. I know you add triangles to each side of the square. However, mine were always difficult to square up. So, I finally figured out a few tricks.
First, I find the center of my triangle and fold right sides together. Then I fold my square right sides out.
That way they nest together to line up.
Pin well. (I used to hate pinning, and yes, it takes a little more time, but it sure saves on accuracy).
Then, here's the trick...Sew with the stretchy triangle on the bottom.
Remember that old line, "Baggy Bottom". It fits many scenarios. Here it kept my triangle from shifting, and the pins kept it lined up with the square.
And here is my very nice square in a square block. The beginning of a very cute table topper to be revealed later.