When the chazzan reaches Modim, the tzibbur should bow together with him the same way they bow during Shemoneh Esray. (ShA 1 – MB 1)
The tzibbur recites Modim D’rabbonon. The chazzan does not need to wait for them to finish; he should continue normally. (MB 3)
The minhag is for the tzibbur to be bowed over for the duration of the Modim D’rabbonon. (Rama 1 – MB 5)
The Chayay Odom says the ikar din is to straighten a little at Hashem’s name until the end, and then bow again. (BeH “V’yaish”)
The Gr”a writes that it is not necessary to stay bowed over except until “sho’ata hu Hashem Elokaynu…” then straighten. (BeH “hakol”)
When the chazzan replaces Birkas Cohanim for “Elokaynu Vaylokay avosaynu…”, the tzibbur should replace “omayn” with “kayn yehi razon”. (ShA 2 – MB 11)
“Borchaynu babrocha” with a passach. (MB 8)
As the chazzan says “Y’vorech’cho Hashem”, he should look towards the haychal. As he says “v’yishm’recha”, he should look towards his right. “Yo’er Hashem…” – look towards the haychal. “ponov aylecha veechunekka” look towards his left. (MB 8)
If the chazzan forgot to say “Elokaynu Vaylokay Avosaynu…” we don’t make him go back. (MB 7)
The minhag is to say Sim Shalom anytime that there would be Birkas Cohanim, otherwise it is Shalom Rav. There are those who connect Sim Shalom with Krias Hatorah as well; they therefore say Sim Shalom at Shabbos Mincha. (Rama 2)
If you by mistake said Shalom Rav at Shacharis you should perhaps go back to say Sim Shalom if you have not yet said the final beracha. But if you, by mistake, said Sim Shalom at Mincha do not go back to correct it. (BeH “aval”)