Sprinkling and pouring will not fit the action of baptism. In fact, sprinkling and pouring are not New Testament baptism at all. They are human substitutes without one iota of divine authority. James Gibbons, a Cardinal in the Catholic Church, wrote: "For several centuries after the establishment of Christianity, baptism was conferred by immersion, but since the twelfth century the practice of baptizing by infusion (pouring) has prevailed in the Catholic Church as this manner is attended by less inconvenience than baptism by immersion: (Faith of our Fathers)
New Testament baptism is immersion for remission of sins and was practiced by the Lord's Church as commanded by Jesus twelve hundred years before man made Catholic church changed it.
Once again the New Testament answers the question clearly. Look at the following scriptures. Matthew 3:5-6, Then went out to him Jerusalem, and all Judea, and all the region round about Jordan, and were baptized of him in Jordan, confessing their sins. Matthew 3:16, And Jesus, when he was baptized, went up straightway out of the water: and, lo, the heavens were opened unto him, and he saw the Spirit of God descending like a dove, and lighting upon him: Mark 1:9-10, And it came to pass in those days, that Jesus came from Nazareth of Galilee, and was baptized of John in Jordan. And straightway coming up out of the water, he saw the heavens opened, and the Spirit like a dove descending upon him: Acts 8:38-39, And he commanded the chariot to stand still: and they went down both into the water, both Philip and the eunuch; and he baptized him. And when they were come up out of the water, the Spirit of the Lord caught away Philip, that the eunuch saw him no more: and he went on his way rejoicing. We see there was a going down into the water and while they were in the water the eunuch was baptized. The apostle Paul when he wrote to the church in Rome said in Romans 6:3-5, Know ye not, that so many of us as were baptized into Jesus Christ were baptized into his death? Therefore, we are buried with him by baptism into death: that like as Christ was raised up from the dead by the glory of the Father, even so we also should walk in newness of life. For if we have been planted together in the likeness of his death, we shall be also [in the likeness] of [his] resurrection: In Colossians 2:12 Paul says, Buried with him in baptism, wherein also ye are risen with [him] through the faith of the operation of God, who hath raised him from the dead. From these scriptures we can see how one is to be baptized. Baptism is a burial, immersion into water being baptized (immerses) into his death and raised up from the dead by the glory of the Father that we might walk in newness of life. Does sprinkling or pouring meet the teaching of the scriptures. Certainly not...
As we look at the New Testament we see that those who have heard the gospel and can believe, repent of their sins and are baptized receive remission of their sins. This answers our three questions. Who - penitent believers, Why - for remission of sins, How - by being buried (immersed) with Christ through baptism into death; that as Christ was raised from the dead through the glory of the Father, so we also might walk in the newness of life.