I have uploaded a new video on the main page about installing a button hole. I am using a #Berninausa B570 QEE sewing machine. Bernina designed an automatic button hole foot but a manual one can be done as well with other models or brands of machines. Each machine has its own instructions and reviewing the manual will give you the best method to sew one.
Button hole feet are all essentially the same. There is a limiting lever for manual machines that is pulled while sliding a gauge to the length of the button hole desired. A automatic button hole does the same thing but it is programmed into the machine.
Button holes are not only useful for garments or clothes, but great to slide ribbon or string through for an ornament, gift tag or to used to slide bunting flags on a string. Give them a try. They are fun and quite useful.