A link to the actual process and a whole bunch of quests:
Planar Flagging Planes of Valor, Storm and Torment are accessible at level 55 without flagging. However you do need to flag if you wish to continue the Planes Progression.
Bastion of Thunder and Halls of Honor are accessible at lvl 62 without flagging. However you do need to flag if you wish to continue the Planes Progression.
This is a map of the planes:
Map of Planes And this is a roadmap of the process:
Big Map of Process