Hey Guys & Gals!
I have a 100% Achievement Guide for this game if you prefer a visual!
At the beginning of the game and you are following the main path, you will eventually come across a Donkey (Hard to miss as he is in your way sort of).
Anyway, just above him is a fenced area, so circle around up north to find a tree and 2 apples.
Simply pick them both up, and feed one of the apples to the donkey to unlock the achievement.
Go to 5:15 in the video if you want a visual look.
Hope this helps!