Currently the most recent information regarding how to install, setup and configure can be found in the DD-WRT wiki. The Wiki is mainly maintained by the DD-WRT community and contains a lot of information, often in several languages. Because DD-WRT has matured over the years take care to check the respective wiki page what version of DD-WRT a manual is written for. In some cases the setup and behavior of DD-WRT differs between the releases.
If you are missing something, please feel free to add information to the wiki or ask users in the forum. Often there are solutions that are not published via the wiki, yet.