From the movie The Lobster, after certain days if they can't find the pair they will change them to animals. How do they change it?
They get sent to the transformation room. Scene 60:
And so he decided to turn her into an animal. A hotel maid helped him and he never understood why she did it. I asked him many times what sort of animal he turned her into, but he always gave the same answer. “That’s none of your concern”. That night he left the hotel, once and for all.
An example of the trope
From a conversation between David and his two friends while at the hotel (roughly 30min into the film):
THE LIMPING MAN: What they do, as soon as you enter the [transformation] room is to wash your body and head really well.
LISPING MAN: How do you know?
THE LIMPING MAN: My uncle used to know a waiter who worked here.
LISPING MAN: No waiters know about the procedure. They're not allowed inside that room.
THE LIMPING MAN: Nevertheless. They peel off the skin, which has become soft due to the water and the soap. Afterwards they remove the heart, the eyes, and other vital organs of the body with a laser or scalpels. Then the procedure changes according to the animal when it's chosen.
DAVID: That makes total sense. I mean, I suppose mammals demand a different kind of work than fowls, for example.
THE LIMPING MAN: Exactly. Afterwards they throw the remaining body parts into the casks outside the hotel's restaurant and distribute the blood to the hospitals in the city.
Assuming the description provided by The Limping Man is true, that's as much detail as the movie provides.