General:
http://pastebin.com/812CAxSU
http://www.maxconsole.com/maxcon_forums/threads/286487-3DS-N3DS-Gateway-Ultra-Guide
https://www.youtube.com/playlist?list=PL1CadovfabPtTXpw4iC_R9MiLMdLOZS3h

Downgrading:
http://www.maxconsole.com/maxcon_forums/threads/286618-GUIDE-Downgrading-your-3DS-from-4-5-9-2-down-to-4-x
http://gbatemp.net/threads/guide-the-emunand-save-data-mega-guide-gbatemp-doesnt-deserve.378491/

Converting 3ds to CIA:
http://gbatemp.net/threads/tutorial-converting-3ds-to-cia-for-dummies.373722/

Installing CIAs:
http://www.maxconsole.com/maxcon_forums/threads/285969-BigBlueMenu-and-CIA-files

Converting .3ds to .3dz to play online:
http://gbatemp.net/threads/scene-roms-online-tutorial-for-dummies.370589/
http://www.maxconsole.com/maxcon_forums/threads/284944-How-to-play-online-TUTORIAL!!!-And-all-things-online!!!

Downgrading without Gateway and installing CFW:
http://gbatemp.net/threads/tutorial-from-9-2-or-any-lower-firmware-to-cfw-4-5-without-gateway.378371/