Use a scrap fabric sample first. When you put on the buttonhole foot, you press the 0 (blue button) that has the buttonhole symbol. The buttonhole symbol should appear on the screen. Put your foot on the pedal and it will start to zig zag sew the left side. Wherever you move that little red slide on the foot, is the length of your buttonhole. When the red slide and the little red line are lined up, press the little reverse symbol and the machine will straight stitch back to the starting point. Then it will zig zag up the right side of the buttonhole. It will bar tack at the end.
Check the buttonhole and if it is the right size, all the rest of the buttonholes will be the same size until you change the setting.