When your character reaches level 48 you will receive a mail in the post from a Lycini Envoy. The mail is the same for Guardians and Defiants.

You can choose to finish off the Mathosian zones, or go straight to Storm Legion at level 48 and start questing there.

The item ‘Strategic Overview of Brevane and Dusken’ should be collected from your mail and then right clicked to start the quest.

The quest will ask you to speak to an NPC near the portal in Sanctum or Meridian. This NPC will offer to teleport you to the Storm Legion lands, you can choose between Brevane or Dusken. It does not matter which you choose first, Brevane and Dusken both start with a level 50-52 zone. I recommend you choose one of them, go do the first zone on that continent and then come back to Hunel Pudillo to be teleported to the other continent and do the first zone there too.

Storm Legion quests are the same for Guardians and Defiants, except for a few quests in Cape Jule/Pelladane. So each Storm Legion zone has only one page per zone, with quests for a specific faction clearly marked as Defiant or Guardian only.