To get this achievement you'll need to collect half of the hats the game has to offer, there are a total of 12 hats. You start the game with 1, receive 1 hat for beating the game and will have 1 more if you paid for the dlc for the game. There is also a hat that you can find if you have purchased the nomes attic DLC. Here is my own video guide that show all of the hat locations along with all of the other collectible / achievements the game has to offer.
Chapter 1Hat #1 - 1:38
Hat #2 - 2:19 (Only if you have the Nomes Attic DLC)
Hat #3 - 5:16
Chapter 2Hat #1 - 7:35
Hat #2 - 9:54
Chapter 3Hat #1 - 12:30
Hat #2 - 17:00
Chapter 4Hat #1 - 20:04
Hat #2 - 21:42