[Koha] Koha Interface

Hello, I know I'm flooding the mailing list but I have another question. I want to really modify the interface of koha OPAC but I don't know where to start. I just don't want to change the logo and headers. I really want to customized my whole OPAC website (change the codes etc). I tried to change the css and tmpl files but I can't see the results. The manual provides little info. I can't understand what the wiki on koha template style guide. Can anyone help? I really don't know where to start. How do you change the interface of koha? What files do I have to change? thanks in advance