Allah Almighty says in the Quran: 

“The fornicatress and the fornicator, flog each of them with a hundred stripes. Let not pity withhold you in their case, in a punishment prescribed by Allaah, if you believe in Allaah and the Last Day. And let a party of the believers witness their punishment” [al-Noor 24:2] 

“And those who accuse chaste women and then do not produce four witnesses – lash them with eighty lashes and do not accept from them testimony ever after. And those are the defiantly disobedient, ” [An-Noor 24:4]

• Hudd punishment – The fixed punishment – is prescribed by Allah  for those acts which Allah has made haram and this punishment is by hitting or by killing.
• The fixed punishment for fornication (consensual sexual intercourse between two persons not married to each other ) is flogging a 100 strikes. As for adultery ( voluntary sexual intercourse between a married man and someone other than his wife or between a married woman and someone other than her husband) – the married person would be stoned to death as commanded by Prophet Muhammad (s)
• The punishment is applicable when the man has coitus with the woman. Any relationship not inclusive of coitus does not mandate the prescribed fixed punishment.
• The punishment is applicable when the act of coitus is witnessed by four male adults who are muslim, sensible, just and free  and who testify in the court. Seeing two people together or nude or in some inappropriate position does not mandate the hudd punishment unless the act of coitus is eye witnessed.
• The punishment is also applicable if the fornicator or the fornicatress confesses to his or her crime four times.
• 100 flogs can be inflicted in one day or over a period of days depending on the health of the one being punished.
• No judge can increase or decrease the punishment
• Punishment has to be delivered publicly in front of a small group of Muslims – minimum three and maximum 1000 Muslims should witness this event.
• Flogging should be inflicted with a leather whip which is neither totally new nor very old. Flogging with a stick is not allowed
• Flogging should be on the fleshy part of the body only affecting the skin and avoiding the head and the sensitive areas. Flogging has to spread over – and should not be done in one place so as to make the flesh come out.
• A man is flogged in a standing position while a woman should be flogged in a sitting position.
• Ibn Mas’ood said:  It is not prescribed in our religion to make the offender lie down or to tie him up or remove his clothing. The companions of the Messenger of Allaah (peace and blessings of Allaah be upon him) flogged offenders but there is no report that any of them made the offender lie down or tied him up or removed his clothing. (Islam QA)
• No one should carry out the hadd punishments without the permission of the ruler. If there is no ruler who rules according to sharee’ah then it is not permissible for the ordinary people to carry out the hadd punishments. Whoever does that is sinning, because carrying out the hadd punishments requires examining the matter and requires shar’i knowledge in order to know the conditions of proof. (Islam QA)
• Accepting this punishment is conditional to Emaan – If you feel pity for the fornicator or fornicatress  - that may lead to not approving the flogging punishment then you don’t believe in Allah and the Akhira ( You leave the fold of Islam)

PUNISHMENT OF FORNICATION AS IMPLEMENT BY RASOOL ALLAH (S) AND HIS COMPANIONS

Bukhari :: Book 8 :: Volume 82 :: Hadith 818

Narrated Zaid bin Khalid Al-Jihani:

I heard the Prophet ordering that an unmarried person guilty of illegal sexual intercourse be flogged one-hundred stripes and be exiled for one year. Umar bin Al-Khattab also exiled such a person, and this tradition is still valid.

FLOGGING THE ONE WHO CONFESSES AND LETTING GO THE ONE WHO  DENIES THE SIN

Dawud :: Book 38 : Hadith 4423

Narrated Sahl ibn Sa’d:

A man came to the Prophet (peace_be_upon_him) and confessed before him that he had committed fornication with a woman whom he named. The Apostle of Allah (peace_be_upon_him) sent for the woman and asked her about it. But she denied that she had committed fornication. So he inflicted the prescribed punishment of flogging on him, and let her go.

FLOGGING WHEN THE SINNER CONFESSES AND DOES NOT RETRACT IT

Malik :: Book 41 : Hadith 41.2.13

Malik related to me from Nafi that Safiyya bint Abi Ubayd informed him that a man who had had intercourse with a virgin slave-girl and made her pregnant was brought to Abu Bakr as-Siddiq. He confessed to fornication, and he was not muhsan. Abu Bakr gave the order and he was flogged with the hadd punishment. Then he was banished to Fadak, (thirty miles from Madina).

Malik spoke about a person who confessed to fornication and then retracted it and said, “I didn’t do it. I said that for such-and-such a reason,” and he mentioned the reason. Malik said, “That is accepted from him and the hadd is not imposed on him. That is because the hadd is what is for Allah, and it is only applied by one of two means, either by a clear proof which establishes guilt or by a confession which is persisted in so that the hadd is imposed. If someone persists in his confession, the hadd is imposed on him.”

NO FLOGGING PUNISHMENT FOR THE RAPE VICTIM

Malik :: Book 41 : Hadith 41.3.15

Malik related to me from Nafi that a slave was in charge of the slaves in the khumus and he forced a slave-girl among those slaves against her will and had intercourse with her. Umar ibn al-Khattab had him flogged and banished him, and he did not flog the slave-girl because the slave had forced her.

NO FLOGGING IF THE SINNER IS TOO WEAK AND COULD DIE WHILE BEING FLOGGED

Muslim :: Book 17 : Hadith 4224

Abd al-Rahman reported that ‘Ali, while delivering the address said: O people, impose the prescribed punishment upon your slaves, those who are married and those not married, for a slave-woman belonging to Allah’s Messenger (may peace be upon him) had committed adultery, and he committed me to flog her. But she had recently given birth to a child and I was afraid that if I flogged her I might kill her. So I mentioned that to Allah’s Apostle (may peace be upon him) and he said: You have done well.

Sources:

Tafsir Ibn Kathir :Explanation of the prescribed punishment of zina : http://www.tafsir.com/default.asp?sid=24&tid=35488

Islam QA: How to flog? http://www.islamqa.com/en/ref/13233/flogging

Who should carry out the hudd punishment for zina? http://www.islamqa.com/en/ref/8980/zina

 Crime and Punishment in Islam: http://www.islamreligion.com/articles/249/

 Explanation of verse 1 and 2 of Surah Noor by Dr.Farhat Hashmi http://farhathashmi.com/dn/Portals/0/audio/explanation/Detailed/taleem-al-quran-05/mp3/Para-18/lesson-05/tqc18-05b.mp3