This simple guide will provide you a full explanation where all 120 stars would be. This guide excludes getting 6 Stars in each 14 courses. Secret Stars are trickier to obtain than you think it would be.

• Flying Cap World (10-13 Stars later, look up in the ceiling)
• Metal Cap World (Find it somewhere in Course 6)
• Invisible Cap World (Drain the moat)
• Peach's Slide (Box; over 21 seconds)
• Peach's Slide beat under 21 seconds
• Get all 8 Red Coins in all 3 Bowser Courses
• Get 100 Coins in all 12 Courses (Get stray coins, kill enemies for coins, get blue coins, get red coins)
• Get all 8 Red Coins in the Aquarium World
• Get all 8 Red Coins in the Rainbow World
• Capture MIPS twice (20 Stars he'll appear, 50 stars he'll appear again in the same place)
• Speak with the Toad on B1 (nearby Course 6 Entrance), 2F and 3F.

You should by then have all 15 Secret Stars.