When the time arrives to recite Birkat Kohanim, the Kohanim are not permitted to start the berachah until the chazan or the gabbai announces “Kohanim.” This is because the Torah states (Numbers 6:23), “Say to them,” denoting that Moshe, who is not a Kohen, must tell the Kohanim to bless Israel. Thus we learn that first the Kohanim must be called, and only afterward do they begin blessing. If only one Kohen ascends the platform, “Kohanim” is not declared, since it is written, “Say to them” (in plural). Thus, the word “Kohanim” is only announced when there are at least two Kohanim present (Shulchan Aruch 128:10). Some authorities conclude that the essence of Birkat Kohanim is when there are at least two Kohanim to perform the blessing (Rabbeinu Peretz brought by the Tur 128; see Aruch HaShulchan 128:9).
The blessing itself is not recited by the Kohanim on their own. Rather, the chazan recites each word and the Kohanim repeat after him. Some chazanim mistakenly recite the words in a quiet voice. That is incorrect. Instead, the chazan‘s voice must be heard clearly by the Kohanim and should be just slightly softer than theirs (see Tzitz Eliezer 14:17; Nesiat Kapayim Kehilchatah 12:3.) Following each verse, the congregation answers Amen.
As we learned (in halachah 1) the awakening of our will to accept Hashem’s blessing awakens the Supreme Will to abundantly bestow blessing upon Israel. In other words, when HaKadosh Baruch Hu created the world, He established that free will was to be a key component. Therefore only after we awaken from below the desire for something positive, a parallel desire from Above awakens to aid us in achieving that thing. Hence, it is necessary that one of the Yisraelim being blessed first declares “Kohanim,” thereby expressing in words our desire to accept Hashem’s blessing, and only then the Kohanim begin to bless. Afterwards as well, the chazan must read every word first, and in doing so, express our desire for every detail of the berachah, following which the Kohanim repeat that word, and HaKadosh Baruch Hu abundantly bestows His blessing upon us (Likutei Halachot LaMaharan, Nesiat Kapayim 3:4).