You will need some sort of dumper or NDS linker, which basically allows you to connect your original cartridge to the device, and then you connect the device to the computer. Just google the various names. I recommend you don't though, other people are dumping roms fine, you'll be wasting your time and money.
He said he already has them, so what's the harm? You'll need a slot 2 flash cart and then the homebrew "NDS Backup Tool" which you can find on google. There are two ways to do it, one where it stores it on your flash cart and one where it transfers it over wifi, the former being the easiest.
