Here is a quick idea of how these blocks are set. I was too lazy to actually SEW one, but I think you can see the theory. For each block you need : the center (this can be square or rectangular), 2 squares of the sashing fabric (they need to match to get the optical illusion) and 2 strips of the dark 'shadow' fabric. You need to audition a few. Grey works the best, and sometimes black is too stark. The width of the strips will match the size of the square. They should be the same throughout the quilt to maintain the same 'shadow'.

One small square on the top right corner and the bottom left corner. You can play around with what sides the strips and squares are one, but it needs to be all the same sides.

Clear as mud? I hope the picture clears it all up!