Flare blitz is great in an advantaged state. If you're looking to hit this move more reliably try using it to chase techs or when the opponent is trying to getup roll. It's good if your foe is getting to trigger happy with projectiles, but sometimes it's hard not to make it real obvious. Its easier to intercept projectiles if you jump and fall then use it, especially if they're charging a projectile or committing to projectile with lots of lag that fb can eat (sheiks needles.) If you want a guaranteed hit, use it if your enemy is recovering onto the stage, and to a lesser extent, if they've used their double jump. It's good to keep the move fresh. I feel like the first time you use it it's almost as powerful as a smash attack, but then the knock back heavily declines, or at least it feels like it. Generally when your opponent is at low percent, its not really worth it since you damage yourself as well.
Sometimes you just gotta say yolo and go for it. Try using it offstage for an extremely early kill (we're talking like 30-40 or even less.) If you're on the stage and you're trying to hit someone who's off towards the side blast zone, be at least two steps away from the ledge before using it so you won't kill yourself instead. Another offstage use is to jump if the opponent is recovering low, flare blitz back towards the stage but just barely below the ledge so you won't snap to it. If all goes well it will hit them mid recovery and stage spike them. It is VERY satisfying.
As many have said before, don't over use it, but don't neglect your instincts either. At times you'll just feel it in your bones and active flare blitz and surprise yourself when it connects. Gotta make risks with Charizard, it pays off.