What n00bs should know
From ROM Hack City
ROM hacking is easy, but you must first be comfortable with the geekier aspects of using a computer. You must be comfortable using command-line utilities. It even helps to be able to program tools when you need one and it turns out it doesn't exist already.
By the way, ROM hacking doesn't apply to just ROMs. For instance, a CD-ROM image, despite having "ROM" in its name, is not really considered a ROM image; it's a CD-ROM image. Likewise, installed computer game files can be subject to the processes of ROM hacking, although no ROM of any kind is involved.
[edit] Is it legal?
The short answer is: ROM hacking is probably legal. While downloading roms may be legal if you own the game, it most assuredly is illegal if you don't.
If you are not distributing nor proffiting off of copyrighted materials, you are probably being legal. Probably is a needed modifier because this hasn't shown up is case law, but take heart in knowing that it most likely never will.
You can read more information at Legality of ROM hacking.
[edit] Why do it?
Reasons vary. Whether you plan on translating a game from, say, English to Spanish, or whether you plan on making a whole new game for you, your friends, or others to enjoy.
[edit] How to do it
How to hack a ROM depends on what you want to do. If you want to change graphics, you need a tool such as Tile Layer Pro, which can easily edit uncompressed graphics for several systems. If you are looking for a tool specifically for a certain game, try searching game name and then what kind of hacking utility you need, like map editor.
On the other hand, Tile Layer Pro will do nothing for you if you want to change a game's text. In that case, you want a script insertion utility such as Atlas, and possibly a hex editor such as Translhextion. If you only want to change a couple of menus or other small bits of text, you don't need a script editor, only the hex editor. No matter how you modify text, you will need the game's TBL file. If you don't have its TBL file, you must make one yourself.
If you want to change the game's sounds or music, you're probably in bad luck. Editors exist only for a handful of games, such as The Legend of Zelda: A Link to the Past.
If you want to change the game's levels, you need a tool to do this. If there isn't already a tool, you need to make one yourself, and again you're on your own.
If you want to edit movement of characters, Graphic Creation/removal/changes and cause and effect elements (Damage). Apparently the tool for this does not exist. Though it can be made with some effort, and a lot of coding.
