The map is quite big, so it can maybe be hard to find all of them first time, here are some extended hints from the hintboard you can find in the map:

• The first headcrab can found in the pools area in the shower area...
  
• The second headcrab can be found somewhere in the movie theater...
  
• The third headcrab can be found on top of a familiar rooftop...
  
• The fourth headcrab can be found inside the arcade, should not be hard to find.
  
• The fifth headcrab is within the second ballpit room, through the tunnels.
  
• The final headcrab is in a macabre coffee shop filled with coffins....

Good luck...
Oh wait, there is one more... A GOLDEN headcrab should appear after finding all of them, unlike normal green headcrabs, it will spawn in a RANDOM location (one of six random spots in the map) Happy hunting! which will unlock the SUPER dev room!


basic map layout: https://i.imgur.com/UjGGA32.png