Custom Command Help!

First, you'll need to know if your ROM has a header or not. You can sometimes figure this out by file size, or you can use a tool like NSRT to check the file.

Then, you need a hex editor. HxD works very well for this purpose. Make a copy of your ROM and open it with HxD. Make sure you've got the right file open, because HxD can change ANY file, and you don't want to mess up something you don't intend.

The addresses listed refer to locations in the ROM, but HxD reads them as direct file locations. Because of that, you need to subtract C00000 from any address to get its corresponding file address. In most cases, that means just dropping the "C" in front, but this will be important if you do more hex editing down the line.

You go to the address specified and look at the bytes therein, which are written in hexadecimal format. Then you change those bytes to what you want.