Paul said he received instructions for the ordinances directly from Jesus Christ:

2 Now I praise you, brethren, that ye remember me in all things, and keep the ordinances as I delivered them to you.

23 For I have received of the Lord that which also I delivered unto you. (1 Corinthians 11:2)

Unfortunately those instructions became lost, and all that remains are its rude reflections. Thankfully, they are found in the Book of Mormon as Jesus gave them:
 

1. By Water

a. Both Were Buried

12 And now it came to pass that Alma took Helam, he being one of the first, and went and stood forth in the water, and cried, saying: O Lord, pour out thy Spirit upon thy servant, that he may do this work with holiness of heart.

13 And when he had said these words, the Spirit of the Lord was upon him, and he said: Helam, I baptize thee, having authority from the Almighty God, as a testimony that ye have entered into a covenant to serve him until you are dead as to the mortal body; and may the Spirit of the Lord be poured out upon you; and may he grant unto you eternal life, through the redemption of Christ, whom he has prepared from the foundation of the world.

14 And after Alma had said these words, both Alma and Helam were buried in the water; and they arose and came forth out of the water rejoicing, being filled with the Spirit.

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.

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. (Mosiah 18)

b. Belong to the Church of God

18 Therefore, Alma did go forth into the water and did baptize them; yea, he did baptize them after the manner he did his brethren in the waters of Mormon; yea, and as many as he did baptize did belong to the church of God; and this because of their belief on the words of Alma . (Mosiah 25)

c. Priests and Teachers Were to Baptize

13 And Alma established a church in the land of Sidom, and consecrated priests and teachers in the land, to baptize unto the Lord whosoever were desirous to be baptized . ( Alma 15)