The recommended way to extend or alter CORE's functionality is via plugins. Documentation on other functionality is available as well but plugin developers should not rely too heavily on it. The plugin API is meant to be quite stable but the underlying functionality behind the API may be subject to change.

Stores already running some version of Fannie may want to start with the Porting documentation. It's more of a step by step approach that makes sense when you already have a bunch of code as opposed to someone who's starting from scratch.