Players receive Battle Stars for leveling up in Fortnite, meaning the more XP you earn from competing in matches and finishing challenges, the more Battle Stars you'll receive. Five Battle Stars are awarded for each level you earn in the Battle Pass — meaning if you reach Level 100, you should have 500 Battle Stars.
There are plenty of ways to earn XP in Fortnite, so long as you spend some time playing the game.
For starters, the better you perform in a match, the more XP you'll be rewarded. Players earn 50 XP for nabbing the first kill in a match and 20 XP for their next seven kills. You'll also get 17 XP per minute for the first 14 minutes of a match, and 25 XP for placing in the top 20, 100 XP for the top 10, and 300 XP for a Victory Royale.
Occasionally, there are also XP coins you can collect in a battle for more XP. Really, the more you play, the more XP you'll be awarded.
While you can't purchase Battle Stars like you can V-Bucks, you'll earn Battle Stars for the levels you purchase in the game. Players can purchase a level upgrade using V-Bucks, so if you're looking for the fastest and easiest way to earn them, this is likely it — though it will require spending some real-world money. Some may consider it cheating but use your own ethics to determine whether or not this is how you want to earn your Battle Stars.