Q- "What is the procedure for a Christian to convert to Islam in order to marry a Sunni Muslim girl?"
A- It is not a correct practice to embrace Islam for the sake of marrying a Muslim girl only. Islam is a composition of certain beliefs and acts. It is a way of life. It is a matter of faith and conviction. In order to become a Muslim, it is necessary to accept all its basic teachings with one's heart and soul. If one's real purpose is only to marry a Muslim girl, and he wants to register himself as a Muslim only because he cannot marry that girl without it, while he does not have faith in the basic beliefs of Islam, he cannot be a Muslim in reality.
Therefore, one should know at the first instance that conversion to Islam is not meant for marrying Muslim girls. One should, first, study the basic Islamic beliefs and teachings, and if he is convinced that they are worth accepting, he should accept Islam for its inherent rightfulness and its intrinsic merits, and not only because he wants to marry a girl.
Now, the basic beliefs one is required to accept while entering into Islam are the following:
(i) God is One. He has neither a partner, nor a son or daughter. He is One in the true sense of the word that has no room for the concept of trinity, or for any other form of camouflage monotheism or a disguised polytheism.
(ii) The Holy Prophet Muhammad, Salla-Allahu alayhi wa sallam, is the last Messenger of Allah after whom no messenger or prophet of Allah (in any sense of the word) will come.
(iii) The Holy Qur'an is the last of the divine books revealed on the Holy Prophet, Salla-Allahu alayhi wa sallam, and all its contents are true.
(iv) The life Hereafter is the eternal life one has to live after his death where he will have to face the fate of his good and evil deeds.
(v) All the teachings given by the Holy Qur'an or by the Holy Prophet, Salla-Allahu alayhi wa sallam, in absolute and unambiguous terms are true and acceptable.
Once a person accepts all these fundamental beliefs as true, both verbally and in his heart, he becomes a Muslim.
Conversion to Islam has no particular procedure, like baptism, etc. As soon as a person accepts the aforementioned beliefs with his heart and soul and professes them verbally, he enters the fold of Islam. It is not necessary that he seeks the mediation of a saint or priest, nor is it a prerequisite to go to a mosque or to an institution for accepting Islam. One can accept Islam on his own. However, it is advisable to go to a learned Muslim who can inform him about the basic beliefs of Islam and can teach him the concise and comprehensive words to express his acceptance of those beliefs.
Normally following sentences are used for that purpose:
"I bear witness that there is no God but Allah, and I bear witness that Muhammad, Salla-Allahu alayhi wa sallam, is His slave and messenger."
"I have believed in Allah and His angels and His book (as they were originally revealed on His prophets) and in His messengers and in the Last Day and in the fact that all the good and bad destinies come from Allah and in being raised alive after death."
A further requirement for a convert Muslim is to free himself from those beliefs of his former faith or religion which are not in line with the Islamic beliefs. For example, a Christian must proclaim that he does no more believe Jesus Christ as the son of God or a part of Godhood. Instead, he accepts that he was a revered Messenger of Allah, and was no more than a messenger, having all the human attributes.