Creating Bundles
To start creating bundles to extend Sculpin, check the Symfony 2 section on Bundles, especially the How to use Best Practices for Structuring Bundles documentation. There really is not much more to extending Sculpin that writing Symfony 2 bundles that hook into Sculpin's event lifecycle.
Register your own bundles as described in the configuration section.
You may find it helpful to look at the existing community extensions for inspiration and examples.