Rescuing Fields of Sheep is the eighth event in the Balloon Garden booth of Free Play. It's unlocked from the very beginning, so this is the easiest way to get there, otherwise you'll eventually reach it through Adventure Mode.
This is a sheep hooking game. To get this achievement you must hook at least one sheep and you must never miss trying to hook a sheep. The best way is to wait until a sheep comes across the bottom row of the field, hook it (this should be easy) and then do nothing for the rest of the level. If you miss, hit start and restart the level.
You must hook at least one sheep -- I already tried not hooking anything and it wasn't enough.
The achievement will pop at the end of the level.