To get this achievement you need 20% of each of the games collectable items. 20% of the wisps and 20% of the coins.
In the entire game there are 300 wisps (20 per level), so 20% of that is 60 (3 levels)
In the entire game there are 75 coins (5 per level), so 20% of that is 15 (3 levels)
This achievement should be pretty natural if you play the game through normally, but there are also secret achievements for 50% and 100% collection, so you may want to aim to do this by the time you've completed the 3rd level.
THE ACHIEVEMENT POPS AFTER THE END OF THE LEVEL, NOT WHEN YOU COLLECT THE FINAL ITEM
Hopefully these videos will help you (I take no credit, I just found them on YouTube)
This is a playlist showing the locations of the collectables on levels 1-13