For information on how to use existing CSL styles with Zotero, see Citation Styles.
Editing CSL 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/.
Mapping of Zotero Variables and Item Types to CSL
Frank Bennett prepared a (currently slightly outdated) mapping of Zotero item types and variables to CSL.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.
Submitting Styles to the Citation Style Repository
Self-hosting CSL Styles
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.