This will be obtainable during Chapter 2.
After a portion of free-roam and learning about a bunch of side content, you'll be tasked with meeting Kaito in the Koi Pond shop. Once you're there, you'll be tasked with beating up some thugs, to then go down into a secret room. This is all story related, so it's unmissable.
As you go down the stairs to the secret room, you find out it's L'Amant, a hidden casino underneath the Koi shop. MAKE A SAVE EITHER BEFORE ENTERING, OR ONCE INSIDE THE CASINO ITSELF You'll need to play a game of Blackjack or Poker to advance the story, meaning once you stop playing whichever one you choose, the story advances, and you're then tasked with leaving L'Amant. You need to double your chips from 300, to 600 before choosing to quit the game you play to advance the story.
You can come to L'Amant any other time, to play and earn more chips, but this trophy requires it to be done in this exact instance.
As for general tips, it's just Blackjack, or Poker. You can google basic rules/knowledge, or let the game show you via its How To Play tutorials. If you have bad luck and run low on chips, or dislike the game and want to try the other. Just quit out of the game, skip cutscenes, then reload the save you made to keep trying.