Hidden Moustache #1
From the pause menu, select 'Options' then 'About' and look for the moustache near the top of the scrolling credits. To collect it (and subsequent moustaches) hit Triangle if you're playing on PS4, or tap the screen on PS Vita. This action will then unlock the rest of the collectibles.
Hidden Moustache #2
After failing a level, a moustache is in the top left of the 'Try Again' screen.
Hidden Moustache #3
Go back to the pause menu, where a moustache has now appeared in the top left corner.
Hidden Moustache #4
From the pause menu select 'Options', where another moustache has appeared in the bottom left corner.
Hidden Moustache #5
On the Trophies screen, unlocked in area 2, a moustache can be found in the top left corner.
Hidden Moustache #6
On The Boom Challenge screen, unlocked in area 3, there is a moustache in the bottom right corner.
Hidden Moustache #7
On the Statistics screen, unlocked in area 4, a moustache is nestled beneath the charts in the top left corner.
Hidden Moustache #8
On The One Rock Challenge screen, unlocked in area 6, the final moustache is located in the top left corner.
Secret Moustache World!
After gathering all eight of these hirsute collectibles and earning the Stash of 'Stache Trophy, a special golden moustache marker will appear in the bottom left corner of the World Map. Moving over it will take you to the Bizarre Portal, allowing you to travel to the Secret Moustache World where everything is backwards and nothing is quite as it seems... enjoy!