It should work the same way as before the quest, with the boat on the worldmap. Is it not there for you? Have you tried to advance the quest further? What quest step are you on?
I've started the Peace with the Luminous Order Cassedy sidequest and don't know what to do since in the quest log it doesn't say where to get the info and I've checked the walkthrough and couldn't find one about the quest anyone who'd help me out?
What info? If you talked with Cassedy and got the quest for the peace route, you just have to take the ship back to the beach of the luminous order, where the next step automatically triggers.
No, not Amaranthe. The "beach" you arrive at when you take the boot south of Central on the worldmap to drive over the sea to the area of the Luminous Order.
Since you went to the area of the Luminous Order before, and met Cassedy there, the boot on the overview map to travel back and forth between the two areas should be in the south of Central.
