Insert the MicroSD into the flashcart, put the flashcart into your DS, turn it on, and boot into the menu to select your game. Using Custom Firmware (Twilight Menu++ on DSi or 3DS)
Look for collections labeled "GhostDawn", "Retro Gaming Preservation", or "No-Intro". These are clean, verified dumps of retail cartridges. nds roms pack best collection install