This is essentially a story-based achievement. You cannot progress in the game without obtaining it.
About halfway through level 4, you will be prompted to press
,
plus
to latch onto one of the tongue rings in the background. The achievement will unlock the second you latch onto that.
As near as I can tell, you cannot obtain this achievement before Level 4! There are no backgrounds to enter before then. Somebody correct me if I am wrong, but I have been checking the backgrounds on the 1st level, and there is nothing (there are no backgrounds on level 2 and 3 due to them not being sidescrollers)
If you are downvoting, I would be interested to know as to why so I can improve the solution. However, I really can't go much beyond "you are guaranteed it in level 4"