The process is quite involved, but Cemu has a detailed game dumping guide that shows you exactly how to do it. In the specific case of Cemu and Breath of the Wild, you’ll need to use a WiiU console with custom firmware to dump a copy of the game from a disc (or from a digital download) to a USB drive. You’ll need a physical copy of this game as well as access to a customized Wii U console to create a game dump.
