To get this trophy, you must collect 23 stars in the AR Challenges, which can be accessed in game throughout Gotham or by selecting it from the main menu. Keep in mind that this is any 46 stars, so you can get stars on DLC packs and those will count as well.
I highly recommend that you wait until you've purchased most or all of the upgrades available before doing this. Having extra functions to your gadgets will help with predator missions, give you more versatility during combat for higher scores, or allow you to handle situations more effectively in the Batmobile.
For those who have it available, I highly recommend doing the Scarecrow missions. Even though it says that it's particularly hard, I found it quite easy (though might be tough if you are colorblind!) These should be able to provide with a quick and easy 9 stars.