For information on how to use existing CSL styles with Zotero, see Citation Styles.
For basic instructions on how to edit CSL styles for Zotero, see the Step-by-step guide. More documentation can be found at http://citationstyles.org/citation-style-language/documentation/.
Frank Bennett prepared a (currently slightly outdated) mapping of Zotero item types and variables to CSL. Note that creator types are not included in these listings. A Zotero extension to create a mapping from a local Zotero installation can be found here.
If you decide to host CSL styles online yourself, serving them with a “text/x-csl” MIME type allows Zotero for Firefox to automatically recognize and install your styles.