Cut a strip to fit the diagonal measurement and a few over, NOT the size of the rectangle.
Sew very carefully, you are working on a diagonal and it will stretch. Be careful and make sure you PRESS without moving your iron.
Then slice the other diagonal.
And cut another black strip. Mine are 1 inch to finish at 1/2 inch. So you do need to sew carefully and straight.
Pretend I didn't cut it already! Here is what it looks like at this point.
Slice down the center.
Insert a strip of black, press carefully. And you have one segment.
Needless to say, Frank was impressed. After I got to this point I realized it was just an asterik block. Duh!