Mosiah 18:11 - 18:15
Mosiah 18:15 And again, Alma took another, and went forth a second time into the water, and baptized him according to the first, only he did not bury himself again in the water.
When they heard that baptism was available to them they got very excited and said they want it.
Mosiah 18:16 And after this manner he did baptize every one that went forth to the place of Mormon; and they were in number about two hundred and four souls; yea, and they were baptized in the waters of Mormon, and were filled with the grace of God.
The first person to commit to this covenant was a man named Helam. As they both stood in the water, Alma asked for the Spirit to come upon him that his heart would have the proper character to allow him to baptized others.
Mosiah 18:17 And they were called the church of God, or the church of Christ, from that time forward. And it came to pass that whosoever was baptized by the power and authority of God was added to his church.
The Spirit of the Lord did come upon him and he said a baptismal prayer. The prayer being that with the authority Alma had from God to baptize that if Helma would be willing to serve the Lord all the days of his life, that Helma would always had the Spirit of the Lord to be his guide and that he would enjoy eternal life because of the sacrifice of Christ who was set apart to accomplish this service to us before the world was created.
Mosiah 18:18 And it came to pass that Alma, having authority from God, ordained priests; even one priest to every fifty of their number did he ordain to preach unto them, and to teach them concerning the things pertaining to the kingdom of God.
With that both Alma and Helam went down together under the water and both came up out of the water happy as can be because they both felt the Spirit of the Lord.
Mosiah 18:19 And he commanded them that they should teach nothing save it were the things which he had taught, and which had been spoken by the mouth of the holy prophets.
Alma baptized the next person but did not go down with him this time.
[Why did Alma baptized both himself and the first convert together? Not sure. Suspect that since Alma was establishing a Church organization and that there was no authority that could baptize him into it. Because baptism is required to join the Church, Alma’s baptism had to be done this way. This is an indication that baptism has two purposes. First for remission of sins and second to join the Church of Christ. We accomplish both when we baptize today. Joseph Smith and Oliver Cowdery was baptized first for remission of sins by John the Baptist. When the Church was organized in 1830, all were baptized again but this time to join the Church.]