For this achievement all you need to do is baa next to the choir in Alvesta. Alvesta can be found by following the map northwest and you will eventually find it, it is a town that is full of tree houses. The choir can be found in the far right tree house and all you need to do is press lb when next to all of them.
It is one of the simplest 100G in almost any game, If you would like a guide of how to find Alvesta or a video guide for the whole achievement I have included my guide below