You need the game ISO (use google). I would use the American version (SLUS_006.62 and SLUS_006.68, CD 1 and CD 2 respectively). Then you need to open the ISO with CD-Mage and extract a big file called PE.IMG. This file contains everything: texts, pictures, backgrounds, fonts, etc.
Additionally you will have to extract the SLUS file too. This file is called SLUS_006.62 and it contains some texts. Also it contains the width of the ingame characters, so we will have to set up the proper width of all those new characters that we may add. I'll show you how to do that later.
Please note that the CD-1 and the CD-2 are both identical, so no need to extract anything from CD-2. We are just going to use CD-1 and then copy the same information into CD-2, only that the SLUS name changes a litte bit in CD-2 (SLUS_006.68).