The answer is... with some difficulty.
Most of our farm animals can withstand a wider range of temperature and conditions than puny humans can. Also, most of these animal enclosures are either too big or too small for humans. Finally, unlike our cattle, humans can be potty trained, which reduces some of the complexity about waste and feed separation.
I would say that instead of looking to how humans treat animals, orcs should take example on how humans treat other humans: Concentration camps were designed to handle as much human density as possible, and to dehumanize those interned, so I would say it is a good template to handle human cattle. I would also look to use exactly the same psychological techniques they used there, to crush all resistance while ensuring some degree of cooperation, including the use of humans as camp supervisors and "special showers" for the cattle that doesn't fit the profile, either physically or mentally.
If you don't want to go that much back, I would suggest to look at old prisons: modern ones tend to hold less humans than you would like, but old ones can be very good indeed.
You can convert farming buildings into concentration camps, but it would probably be cheaper to torn them down and build them directly. If its needed, the minimal modifications would be:
- You need the buildings to be climatized, specially on hotter/colder climates. They not need to be confortable, but at least be kept in the 12-27ºC range.
- You need to separate the feeding area, the sleeping area and the cleaning area. Unlike what it sounds, this is an advantage vs cattle (where you would have the three things on each room), as you can rotate shifts for eating, and as I said, humans can be potty trained, so you don't need a latrine for each human.
- You will need a separated maternity ward (because humans, unlike cattle, cannot fend for themselves after childbirth). This maternity ward does not need to be much better than the rest of the building but to ensure infant mortality is kept to a minimum I would recommend to emulate a hospital environment.
Slaughterhouses, on the other hand, can probably be used almost the same, although you probably will want to change how to kill humans, as it is designed for bigger/stouter animals and is way too fast if you want them to suffer. The other equipment (saws, grinders, etc), can be used on humans as it was on pigs.