For example, this chapter: http://david.rothlis.net/emacs/customize_c.html

walks you through this example: What command is run when I press TAB? How do I find out? How do I find its (elisp) source code? How do I find the documentation for built-in elisp functions or macros or "special forms"? All culminating in figuring out (from the elisp source code) which variable you can set to customise your indentation level.