You must re-play the first prologue mission. when you get outside the building, have a friend invite you to a online session, then just fly to the area i show in the video! its very simple, but make sure anyone who wants to see north yankton joins the game from the prolouge mission!
For some reason, some forum members at various GTA fansites are upset that this glitch is being shared with the public. Like, we're talking pure fury from these people because the above video exists and is now being shared with everyone. I'm serious! They're totally for real and super cereal (click that link, seriously) about not wanting this information to get out to anyone! Oh well.