For a square in the square block?
Cut TWO 4" (and a hair) squares, cut each in half ONCE on the diagonal, attach to the four sides of your inside square.
I like to cut mine oversized, and would probably cut them at least 4.25 if not bigger. I like the floating look.
I sew two on opposite sides of the square.
Just remember when/if you cut them oversized, they will overhang the edges.
After you sew the first two on, you can trim the sizes even with the inside square.
Now add the other two sides the same way.
The formula to figure this is out is to take the FINISHED side measurement, divide by 1.414, and add .5" for a seam allowance.
5" / 1.414 + .5" = 4.03" (that's the "hair")
The 1.414 is essentially figuring out what the diagonal of the background square would be.
Which, btw, will create a block that is FINISHED 7" (and a hair) block.
The exact formula is 5" x 1.414 = 7.07" but if the inside square might be floating a bit, the final block would be a bit bigger.