Need;
Rocket Boots Xtreme (Lite if you're clothy)
Slowfall
Waterwalking
100% Mount
Decent Wall jumping
Swiftness Potion if you're not a druid, hunter or shaman (supposedly not required but highly recommended)

IF YOU ARE A HORDE PLAYER;
Rumor has it that this does NOT work for Horde players because you res at a different graveyard. I don't play Horde so I really can't tell, but feel free to give it a shot regardless.

This was originally discovered by Arrrdolf. I just added a few more tips and details

Arrrdolf said that this was one of the most difficult exploits he knows of, and I completely agree, this was a challenge to pull off. Disconnecting or logging off to take a break will put you back in Darnasuss and back into the real WoW. So if you're going to attempt this, you have to do it all on one go without logging off, or else you will have to try again.

TO GET OUT OF SECONDWORLD; Just disconnect.

Music;
Radiohead - Karma Police
U2 - Sunday Bloody Sunday
Radiohead - High and Dry

Good luck if you're going to try it, and I hope you do, because these are the last times we see the Old Hyjal :3