Start by just enjoying the story. The game mixes up linear levels with open-ended ones, and has an emphasis on exploration over combat (though combat is still a big part of the game). To minimize time to platinum, it is advised to start on Survival difficulty immediately! It's not too hard and even without much experience with the game, it's very doable. Refer to the trophy description of Ultimate Survivor for details.
If you're really unsure of your ability to do Survivor difficulty immediately, know that starting the game on any other difficulty will require that you finish the game one more time (making for technically three full playthroughs).
Of course, it is definitely a good idea to mix in some side content in between the story missions, otherwise Step 2 may be quite the annoying grind. So pick up collectables if you find them, check each areas challenges and keep them in mind while going through them, and just try to clear out as much of the world as you feel like in this stage.
You'll also probably get a bunch of the combat-related and miscellaneous trophies in this step, as well. There are no strictly missable trophies, because of Chapter Select (called Expeditions in the game's Main Menu), but do look out for some of the buggy ones! Update: There is one missable trophy, Avid Shopper, for which you may not buy the Crafting Tool, nor the Enhancement Tool. Check the trophy description for details.
Note about trophy bugs: At launch, just about every trophy was bugged. Subsequent patches by Square Enix were meant to resolve all trophy problems, but they didn't quite. The trophies tagged "buggy" are ones to look out for, since they have been reported as not unlocking for some, long after the latest patch. But in all likelihood, just about every single trophy has a small chance of not unlocking when the requirements are met. Unfortunately, not much can be done about this, except planning out when you'll receive a certain trophy, and cloud saving right before you do. It would be a tedious process, and the trophy may not even unlock on your second attempt. On the other hand, it is perfectly possible (and likely, even) to go through the entire experience without any trophy bugging out. So how cautious you want to be is up to you.
