When you ask how, you lead me to believe that you are looking for the writing techniques he used to portray these horrors.
Characterization is used as he demonstrates Winston's insecurity about Big Brother watching through the telescreens and the Thought Police as he guards his conversations among society. Winston's desire to rebel and hang on to truth is revealed in these moments of characterization as well.
Orwell built suspense to make you wonder what is going to happen next. In the relationship between Winston and Julia, they go to great lengths by travelling different ways to meeting places. It makes the reader wonder if they are going to get caught. When they finally do get caught, it is in the place we least expect, where we thought they were safe: Mr. Charrington's apartment.
Orwell uses foreshadowing in the relationship between he and Julia. They discuss the fact that they will ultimately be caught for what they are doing.
Orwell creates a mood that shifts. In each book, there is a differing mood. Horror makes the biggest impact on someone who is completely comfortable. When Winston and Julia are lying in bed in the apartment above Mr. Charrington's reading Goldstein's work, we feel the ease of the situation. Julia has fallen asleep. It is completely comfortable. After being caught the mood shifts to horror and continues that way throughout the third book while Winston is tortured.
1984 is a big set up. Orwell writes a comedy in the bleakest terms. Comedy, as you know, focuses mainly on the flaws of the society; a tragedy focuses on the individual. Orwell wants to dispel any Romantic or tragic notions that the individual has any chance of freedom or rebellion against the modern totalitarian state. In the end, the state will crush the individual through the following:
torture: pain is the last thing and individual will feel if it tries to rebel. Pure, physical, unadulterated pain.
police-state surveillance: an individual's freedom is limited through profiling and surveillance (both technological and human)
propaganda: an individual cannot rebel if an individual is uninformed, disorganized, and uneducated. The Ministry of Truth blurs the lines between truth and untruth, between persons and unpersons, and between the past, present, and future.
war: individual rebellion is squashed because of state-wide war. Nationalism is at its peak, and war keeps the masses focused on the war effort instead of their own suffering and poverty.
no core: individual rebellion is hopeless when an individual has not core meaning in his life: family, religion, or friendships. The state keeps everyone alienated: physically, spiritually, and emotionally. There is no rebellion if there is no trust.
In my mind, the most horrific aspect conveyed through Orwell's work is the lack of a private realm. The government as devised by Big Brother is one that negates the realm of the private, making everything public. Through constant propaganda, the lack of privacy, complete control and monitoring over human interactions, and ensuring total control through intelligence operations, the totalitarian society is seen in the most awful of lights. When sex and love have to be seen as political actions because of their private nature, things have become quite unbearable. The idea of a government being able to recant one's own beliefs through the manipulation of a person's fear, such as Winston and rats, is quite horrifying and one where individual privacy has long since been replaced by the realm of the public.