You'll need to play Firestarter on Overkill (or above) and complete day 1.
After that, in day 2 you will need to go behind a wooden door next to the computers and open gratings.
There will be a goat, take it and throw it to the van. Then you will receive Achievment.

Thanks for reading, I hope this simple guide will help you!
(Im using PDTH Hud on screenshots above)