"And as I have prayed among you even so shall ye pray in my church, among my people who do repent and are baptized in my name. Behold I am the light; I have set an example for you."
The "prayer" referred to here is the sacrament prayer. There is one among the twelve disciples who is given power to dedicate the emblems of the sacrament. But all of them are to do likewise in the Church in the future.
He has provided the example for them to follow. He has taught them how, and then demonstrated how. He has explained why. Now He tells them to do "even so" in His church.
But notice once again the Lord defines His church. It is those who "do repent and are baptized in [His] name." This ordinance is not for those who are casual investigators of His doctrine. It is not for those who, though baptized, have not repented. It is reserved for those having the proper qualifications. Without having done these things first, the sacrament of the Lord's body and blood are eaten and drank to their condemnation. Instead of it being a testimony before the Father of their faithfulness, it becomes a testimony of their unfaithfulness.
Notice the Lord explains His role as "light." He "set an example for you" and therefore is "the light." The "light" is the guide. It is the pattern. It shows you the way to go. He has done that in word and in action, so that He can claim to be the "light" for those to follow.
In turn, He has told these disciples they must also become "a light unto the people." (3 Nephi 15: 12.) To accomplish that it is essential they must "set an example for" them; meaning that they follow as the Lord has led them. Not an example of vainglory or superiority. Instead to meekly do as He has bidden them to do. Adding nothing, leaving nothing undone. Honing in on the things He would have done, and going about to do them.
The "light" must reflect the Lord's teachings and the Father's will. Otherwise it is darkness. A light cannot shine as His if it is distorted to reflect credit upon the man rather than the True Light, our Lord. When an erring soul entertains mistaken admiration for a man, they are damned. (D&C 76: 99-101.) Therefore, if a person is called upon to be a light, they cannot seek to attract notice for themselves.
The Lord saves. Messengers sent by Him point to Him. They mirror His acts, teachings and practices. They do not seek their own will, but only the will of Him who sends them.
Throughout this visit between the Lord and the Nephites, we are given an extended view of how the Lord establishes His church and doctrine. So long as it is followed, it has the power to allow mankind to always have His Spirit to be with the followers. However, when they depart from the practices and keep merely a form of godliness, they lose the power He sends to us.
These teachings are important enough for the Lord to dwell on, and Mormon to etch them into metal plates. They should be carefully studied, explicitly followed, and greatly appreciated.