Altering anything other than TXDs is really the same as in the PC version but PS2 TXDs are just very weird (although I've come close to understanding them completely). I did in fact do some TXD mods some years ago, but it took me some time. There are tutorials out there that help you with modding TXDs, which you could try out (I did the mods myself, so I don't know if these tutorials are good).
What's rather annoying, is building an ISO everytime you mod the game and then either burn it or transfer it to your harddrive and play via hdloader or something similar. In any case you need a way to run unsigned software on your console, there are a few methods out there...