The button goes on the end, and tells the foot how big the buttonhole needs to be. A smaller button will move the placement of the plastic guides so the hole ends up smaller.
To make a buttonhole, put the button for which you are making the hole in between the plastic holders.
Then remove your normal presser foot and replace it with the buttonhole attachment.
Pick a buttonhole stitch. On
Ready to go!
True fact: I hold my breath each time I make a buttonhole, still! I think it’s a habit.