Custom authentication module

I am trying to create a custom authentication module that authenticates 
against a mysql database, but have found no information about creating 
such a module. I found some pointers to making a custom authentication 
modules by using the odbc module and just modifying it as well as many 
people who, although trying to help, did not answer the question of 
'what is needed to create a authentication module' by pointing to 
external authentication scripts. Would it be wise to just copy the 
internal authentication module, rename it, and edit it as I see fit? 
What functions are required and which ones are used for internal use?
Niels