Not sure the hows are within the scope of this thread (doubt Thar minds, but hey)...still, these are the steps.
1. Buy an SD Card (or use the one that comes with 3DS/XL)
2. Put it into your PC's SD Card port (assuming you have one...if not, out of luck)
3. Go to My Computer > USB Drive (might be G: or E: ) > DCIM
4. Make a new folder, title it something like...idk, "Music" or some BS like that
5. Get all your tracks (preferrably mp3 files; I haven't tried any other kind but I believe 3DS accepts AAC) and paste them into Music folder
6. Eject SD Card from PC
7. Place SD Card into 3DS, go to 3DS Music Playe ror w/e the app is called, enjoy sexy music