Looking for information on how to create a HANS romfs undub

I would like to learn to patch english text to a Japanese game with HANS's romfs feature. However, after various searches, I still haven't found much information about this.
I would really appreciate if someone could point me in the right direction or list the things I need.

Note: I am interested in making the romfs file myself, if it wasn't clear. I already know how to use someone else's.