This is a collection of AO3 works and other resources for creating fan works. (See also the Fanwork Research & Reference Guides tag.)
Some fandoms have wonderful on-line communities (e.g. The Harry Potter Lexicon and Marvel wikis) devoted to creating canon references. Many more do not, but fans still have created deeply thought out essays, maps, tables, histories, and other world building. Hopefully, you can find them here.
AO3 allows custom formatting using limited HTML and CSS (workskins). This collection has a number of formatting "how-to's" in addition to the AO3 docs below.
Note: Works added as bookmarks do not add their tags or fandoms to the collection. Please add your own works