Hex edit any rom but as for a guide you will have to figure that out on your own (it's different from rom to rom) unless your a programmer that is generally a no no
Graphics.. there are plenty of general tools over at the site mentioned
There are specific game hackers around that are more user friendly that just about anyone can use again at the same site mentioned
Done plenty of rom hacks myself but all using the above I aint no programmer so I'm as limited as anyone, it's not very often that a game is going to be simple to mod in any way n the even the stuff I have done in hex was very basic text edits
The real hackers use assembly code (pfft no idea, go to uni for a few years then start hacking)
If you are talking about actual carts then you need a hackers hardware kit to transfure the rom data from the rom chip on the pcb board
If you have a SNES CD console attachment you just put the roms directly on the disc (a certain format may be used depending on the kit you have)
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