Square in a Square Unit
This method requires a ruler that has a quarter inch line on the edge.
I use Doug Leko's Simple Folded Corner ruler.
Lay the small square in one corner face down on the large square. Using a ruler, place the quarter inch line on the small square corner to corner. Cut away the corner. Sew you have just made the quarter inch seam allowance. Sew corner to corner. Press open.
Rotate the block to the right. Place the small square on top of the large square face down. Cut away as in step one. Sew then press open.
Rotate the block to the right. Cut the corner as in the previous steps. Sew, and press open. When the corners are pressed open the block should always measure the original large block size.
Repeat the process for the remaining corner. Press open well, then square up.