Creating a New Style
It is often easier to modify an existing bibliographic style that closely resembles a style you need than it is to create a new style from scratch. (See Modifying Style Templates). If you do wish to create a new style, Creating an Author Date Style gives you a general overview of the process.
Creating a bibliographic style involves building templates for the in-text citations and the bibliographic references to mimic how you want them to appear in your paper. Then adjust the various options to be sure that authors, titles, pages, and other fields are being formatted according to your requirements. Creating an Author Date Style guides you through the creation of a fictitious author-date type of style.