Well I'm working a little on some PS1 game, but assuming things are relatively similar:
-make a list of the types of changes you want to make
-check the internet for what others have found about the game, offsets and such
-figure out how to modify the data in the game so that it doesn't break (filesystem where you extract/insert files with program X that takes care of whatever checksums the image thing has? or something else?)
-if no one knows anything, use a debugger (kingcom made something interesting for PCSX2) and figure out the offsets yourself. you can also search for sequences of stats in files extracted from the image - if things are stored simply in a sequence, you can change some things this way alone (and long sequences are almost always unique in the data).
-you may end up knowing how to change some types of things but not others
Hex editor allows you to open files and show/modify everything inside.