Wherever [they recite the blessing], the priests are not permitted to add other blessings - e.g., "May God, Lord of your fathers, increase your numbers a thousandfold" (Deuteronomy 11:11) - in addition to the three verses [of the priestly blessing]. [These additions may not be made] either silently or out loud, as [Deuteronomy 4:2] states: "Do not add to the matter."
While a priest is ascending to the duchan, he recites [the following prayer] when he leaves his place to ascend:
May it be Your will, God, our Lord and Lord of our fathers, that this blessing which You have commanded us to bless Your people, Israel, be a perfect blessing, that it not be marred by obstacles or iniquity, from now until eternity.
Before he turns to bless the community, [a priest] should recite the blessing:
Blessed are You, God, our Lord, King of the universe, who has sanctified us with the holiness of Aharon, and commanded us to bless His people Israel with love.
Afterwards, he turns his face to the community and begins reciting the priestly blessings. When he turns his face from the community after completing [the recitation] of the blessings, he recites [the following]:
We have carried out that which You have decreed upon us. Deal with us as You have promised us: Look down from Your abode, from the heavens, and bless Your people, Israel.