Put your buttonhole foot on your machine. (If you don't have a buttonhole foot sewvacdirect.com is a good place to look for one.)
Thread your machine and pull the needle and bobbin threads to the left. (I used black thread in my bobbin and white thread for the needle so it would be easy to see which was which.)
In the meantime mark your buttonhole. Many patterns have the buttonholes marked already but it's easy enough to do yourself. Using a pencil make a vertical "I" shaped mark the same height as the button you're planning to use.