100:11 People standing behind the kohanim are not included in the blessing, unless circumstances compelled them to stand there. The people on the sides in front of the kohanim are included in the blessing. (Regarding those to the sides exactly parallel with the kohanim, see Mishnah Brurah 128:92.) In a place where the aron sticks out from the wall, if the people standing by that wall are at the sides and behind the kohanim, they need to leave that place and stand where they will be at least on the sides in front of the kohanim. If this is not possible, they are like those compelled by circumstances beyond their control and they are therefore included in the blessing.
100:12 The shliach tzibbur reads the birkas kohanim (the “priestly blessing”) aloud, word by word, and the kohanim repeat each word after him until they have completed the first verse, at which point the congregation responds with Amen. The same procedure is followed for the second verse and the third verse. The shliach tzibbur should not recite the words by heart; he should use a siddur so as not to make a mistake. He may also say Amen after each verse and it is not considered an interruption because it’s for the sake of the prayers. The kohanim turn south and north (i.e., left and right because we assume that they are facing west) on the following words: yevarechecha (bless you), v’yishmarecha (and keep you), eilecha (upon you), vichuneka (and be gracious to you), eilecha (upon you) and l’cha (to you). These words refer to all those present, so the kohanim turn themselves to the sides in order to bless everyone. They also turn on the word shalom (peace), because this is the conclusion of the blessing. While the kohanim prolong the chanting of the last word of each verse, namely v’yishmarecha, vichuneka and shalom, the congregation reads the prayer “Ribono shel olam…” (“Master of the universe…”). (The part the kohanim prolong should be the last syllable; prolonging the penultimate syllable – as some do – is a mistake – MB 128:169.) The one reading the words to the kohanim, even if he isn’t the shliach tzibbur, does not say this prayer because it would be foolish. If he is the shliach tzibbur, he certainly shouldn’t say it because it would constitute an interruption. The kohanim shouldn’t sing any tune other than the special tune for birkas kohanim; to do otherwise would be ridiculous.