In order to put .flv videos on your ds download it from youtube
then download vlc media player from http://www.videolan.org/vlc/
open the .flv video from the player you downloaded and installed on your computer
in the player click the "file" tab
click the "wizard..." choise
click the...
btw i feel this a good time to remind you guys that sea of thieves was made by rare, who also made DKC1-3 and DK64, alongside banjo kazooie and conker's bad fur day