This move takes a while to start up so setting it up is not really guaranteed. You can use it on recovering opponents, you can use it while recovering. Other than those situations, the actual mindgames are up to you. If your opponent's offensive maneuvers become predictable, bait/wait for such an attack, be sure they will commit to it, and in those cases you can even hit them with a well-charged shieldbreaker thrust. This depends on how well you read your opponent, it's no guarantee.
Another situation is if your opponent likes to roll behind you when up close to avoid attacks, you can use B to turn yourself around, charge a bit, and let out shieldbreaker when they stand back up after dodge-rolling behind you. Dancing Blade is also extremely useful for this situation, and comes out faster.
If you DO break your opponents shield, give them a fully charged, tipped shieldbreaker while they are stunned for the KO. It has the best knockback of Marth's moves (someone correct me if I'm wrong? Fsmash charged & tipped would work very well also).
Obviously if you face an opponent who relies heavily on shielding as defense, bait them out with some kind of attack you've already used a few times that they've shielded before. Then do a similar offensive pattern but back off a bit and shieldbreaker when the shield goes up.
There's no specific, textbook way to mindgame anyone. Try taking risks with shieldbreaker (IN FRIENDLIES/PRACTICE) to see where you can make it hit, often you pull it off when your opponent least expects it. Because of this I'd say use it sparingly - it has great range and can catch your opponent offguard if they haven't seen you using it in the match yet.