In 3 Nephi 12:1, we learn that “the number of them who had been called, and received power and authority to baptize, was twelve.” After calling them, Jesus instructs them on how to baptize. The instructions are on the form. The qualifications had been known at least since Alma’s use of baptism as both the cleansing symbol and the symbol of entrance into the church.
Perhaps there had been some discussions about how baptisms were to be performed, and that is why the details were given (see verse 28). However, most important is the prayer that is to be used. It ends in the name of the Father, and of the Son, and of the Holy Ghost. That clear designation of the Godhood meant that the Nephites would, from this point forward, expand their understanding of the nature of their God.