The Square in a Square block is put together like the Snowball block and the Flying Geese block. In my sample, I've used a 4.5" square and four 2.5" squares. You will cut the large square to the unfinished size you want the block to be. To determine the size for the small squares, divide the size of the large square by 2 and add .25". For example, if your block will finish at 8", the large square will be cut 8.5" square and the small squares will be cut at 4.5" square (8.5 / 2 = 4.25 + .25 = 4.5).
Mark a diagonal line from corner to corner on the back of the four small squares
Place 2 of the small squares on opposite corners of the large square, right sides together, and sew close to the marked line in what will be the seam allowance.
Match up the corners of the small squares and press.
Open the small square up and trim .25" from stitching.
Here is what it looks like from the back.
Repeat the process for the other 2 corners of the large square. Here's a little trick - after sewing close to the marked line, sew another seam about .5" from the first one.
Match the corners together and press.
Open up the small squares and cut in between the two stitching lines.
Now you have a bonus little Half Square Triangle block! You could incorporate it into another part of the quilt or save them for a scrappy quilt.