Chapter 12, “The Administration of Holy Baptism” pp. 86-91
Walther’s comments on baptism begin with a defense of some variation in the wording used. For instance, some would object to inserting the word “God” as in, “God, the Father and God the Son and God the Holy Spirit.” This kind of variation in the words of the rite do not make it invalid. However, Walther prefers the pastor stay as close as possible to the wording in Matthew 28.
Baptism is performed in the authority of God. The pastor performs the act of God, giving this washing to man.
What of the water? On p. 88 Walther quotes that water is required, not some other substance, but that it does not need to be any special water.
Baptism can be practiced by immersion, pouring, or sprinkling. The Greek verb indicates a ceremonial washing, not an immersion.