Sewing a buttonhole now days is a snap! Almost every machine has this feature built in and it's as easy as 1, 2, 3. I always suggest doing a practice run on a scrap piece of fabric. This will ensure that your threading and tension are all correct.
I sew on a Kenmore, it features a buttonhole sensor and came with this special foot. It does all the math and work for you!!
Pull the buttonhole lever down as far as it will go. This is the black tab on the left center of the photo. This lever will ensure the proper sizing of you buttonhole.
Center your fabric and press "start" or begin sewing.
Almost done. Grab your seam ripper and slowly push up the center to the buttonhole creating the opening for the button to slide through.