Nephi turns the negative statement into a positive exhortation. Where he has indicated that the devil teaches men not to pray, it is because the spirit entices men to pray that they should rather “pray always.” Nephi begins a discourse on the nature of prayer. He ties prayer to the main topic of his discourse, the things that one must do. Note the very specific language: “ye must not perform anything unto the Lord….” Thus part the prayer to find the will of the Spirit should precede performances.
In Nephi’s context, this has a dual meaning. Surely he intends this as a general lesson on prayer, which would be required and effective even for those of his people who do not enter “the way” (baptism). In addition, however, Nephi is linking this understanding of a standard practice of prayer (known from the Old World tradition) to this new covenant of baptism, and the use of prayer to find out what one should do. What Nephi is doing is using a known context to teach about a new context. The known mode of prayer continues, and Nephi intends that this becomes the example for the way and reason for praying for the guidance of the Spirit.
It would appear that Nephi’s people may have had the same problem with prayer that later developed in Jerusalem, and against which Christ proclaimed - that of prayer in public for the praise of the public (Matt. 6:5 And when thou prayest, thou shalt not be as the hypocrites are: for they love to pray standing in the synagogues and in the corners of the streets, that they may be seen of men. Verily I say unto you, They have their reward.)
If this is the situation, the known mode of prayer has become separated from an individual/spiritual context and ritualized into a formal, public, mode of discourse. Thus Nephi can use the concept of a known form of prayer, consecrating ones actions and performances to the Lord, but adapt that known mode to begin to teach his people about the necessity of individual prayer for those individual things that one must do or perform that come not as prescribed actions, but as the whisperings of the Spirit.
Rhetorical: This verse closes the chapter, but the expected close of the discourse is missing. Contrast this very abrupt ending with the more formal closure to the previous discourse in 2 Nephi 31: