You need to collect 40 stars in total.
You can get 5 stars in a level so we're going to play 8 levels
In my video I go through the levels and show where the stars are
level 1: 1:05
Level 2: 2:35
Level 3: 4:10
Level 4: 5:45
Level 5: 6:45
Level 6: 8:20
Boss: 9:55
Level 7: 10:20
Level 8: 11:55
Achievement Walkthrough: