This is very simple. All you need is:
--An SD card
A computer
--A DSi or DSiXL
--Web browser for computer-Mozilla Firefox<p> Ok. 1, Insert the SD Card into a compatible slot
2, Download Mozilla Firefox from firefox.com [If you already have it installed, skip to step 4]
3. Finish the installation and open the browser.
4. Next, go to the top where you will find 'tools' . Open the drop down menu and select 'add-ons' 5. Select 'see more add-ons' after opening the menu from step 4.
6. Now, search in the box for 'downloads' and the first result should be what you want. Select and install it, following the instructions.
7. Your browser will reopen, now go to youtube and search for what you want.
-SECOND PART-
having found what you want, select the download icon in the upper toolbar and click the [v] arrow, then 'download' Make sure the filename ends in .flv or.mp4 .
After the download fineshes, insert the SD card [if not already] and 'copy the containing folder' to the SD card. [DO NOT: Put it in a folder. It should be outside everything else.
After you finish, go to the DSI Sound and check to see if its there, if so, Congratulations!
Simple:
SD card
Itunes
Download a MP3 song.
Click on it until itunes opens.
Right click on the song and find "Create ACC file(however it's spelled)".
It'll convert to MP4.
Drag it to your SD card.
Or
Get a Music converter.