I feel that I must let you know that dyslexic I am . So math has always been a problem with me and some times with origami.
This is done with a half square, (1:2). You fold one of the short sides in ( this is an arbitrary fold, depending on the size it can be about a ¼ inch, it does not matter very much). Then you divide the remaining rectangle into 6ths. Be sure follow the valley and mountain folds.