I´m planning on recording videos to upload to Youtube, but I don´t know how to actually record the videos. Does anyone know how to record videos on a 3DS XL or a DSI XL?
You have to use a capture card. I'm not entirely sure how they work but you plug it into a device (from 3DS to Playstation 4, there's different types for different systems) and then to your computer and record the footage. They usually cost around 80-300 dollars (from what i've seen on google, of course it varies depending on the brand and how well it works). Yeah, they're really expensive. You might be able to find a cheaper one though.
