This is the most widely used manual for academic and professional legal citation in the US. Useful tip: check the Bluebook's "Quick Reference" pages on the inside covers for examples, with those in the front for law reviews and examples in the back for courts. Minor updates provided online by the publisher.
Database that allows you to search for the meaning of abbreviations for English language legal publications, from the British Isles, the Commonwealth and the United States, including those covering international and comparative law.
Work from a model. Search for the resource you need to cite in a law review database to see how another article cited it. As authors of the Bluebook, law reviews from Columbia, Harvard, Penn, and Yale should provide good models.
Work from analogy. Try to locate a similar type of authority that is cited and use that citation form as a template.