The slaughterhouse can be purchased in the shop at level 29 and up, and allows you to produce animal parts, which you can process into other goods, for example at the butcher shop or tannery.
The slaughter enclosure can be purchased in the shop for 100 Gold Bars, but can also be acquired by collecting the card pieces (from boosters, NPCs, friendship cards, etc.).
The space in the slaughterhouse enclosure is limited (12 fields). In the beginning, you will only have enough room for a cow, but there are additional expansions (+8 fields each) available in the shop for 10 Gold Bars apiece.
Animals that can be slaughtered will have a meat icon in their context menu. Clicking on it will open a window with information about what slaughtering gives:
Clicking on Slaughter Now will allow you to move the animal to your slaughter enclosure. Once one or more animals have been placed into the enclosure, a timer will start, depending on the animal species.
Once the timer has expired, a pin will appear over the animal, and the animal parts can be collected.
These parts can be sold either straight from the warehouse, or saved to be processed into new products.