either you point a webcam at it or install a capture unit into it (or get one with one pre-installed)

2s on google would've solved that question.

/edit: literally the first entry on google: http://3dscapture.com/ds/

Getting a capture device is expensive, but the only good way to actually record off of your DS. I'd say you're better off getting the 3DS version instead.

NDS capture is quite inexpensive in comparison to 3DS. If you are never going need 3DS capture, NDS is much cheapter.

Recording on PC with a DS emulator is the easiest way for now.

A few popular DS games are available on the Wii U

If there was an FAQ page that contained RNG and DS Capture cards, we could just paste a link for like half the topics made on this board