square, this is the same size as your desired block before it is finished and 4- 2 1/2" squares. These squares are half the size of the block before finishing.
(Another example if you wanted a 6 " finished block you would need 1- 6 1/2" square and 4- 3 1/2" squares.) You always divide the finished block size in half and add seam allowance to get your smaller square dimension.
Draw a diagonal line on the wrong side of each of the 2 1/2" squares.
Lay two squares with the diagonal lines going in the same direction on your large square.
Sew on the two drawn lines.
Press the triangles open to make sure everything is square.
Lay the triangle back down and trim to a 1/4".
Place the other two drawn squares on your block making sure the diagonal line is in the proper position on each square, and sew on diagonal line. After sewing press and trim.
The Square in a Square now measures 4 1/2" and will finish 4".
Have a fun day and enjoy the journey!