Instructions
Download the Nintendo DS emulator of your choice. The Emulator Zone (http://www.emulator-zone.com/doc.php/nds/) is an excellent resource for downloading not only Nintendo DS emulators, but emulators for other platforms as well.
Download the roms of your choice. Keep in mind that downloading roms for which you don't legally own a physical copy is illegal. Keep the rom file in its original zip format, as the emulator will be able to use it even though it is compressed.
Install the emulator you've downloaded. Open the "file" menu and select the "load image" or "load rom" option. After checking to make sure that your controls are configured (this can often be found in the "options" or "configuration" sections), load the rom and test your control scheme.