c by Torben Dannhauer
The process of populating the new module with content/functionality is fairly straight forward and after learning how the Horde API' s work, you should be able to produce new modules in a minimum amount of time.
Jan Schneider talked about this at FOSDEM 2005. You can find his slides at http://www.horde.org/papers/fosdem2005/.
The new module's functionality could by separated in the following aspects: