100:7 When the shliach tzibbur starts saying “R’tzei…,” all the kohanim must leave their places to go duchen. (If they don’t get up at R’tzei, they lose the opportunity to duchen – OC 128:8 – see Bi’ur Halacha there, s.v. aval im, re: whether or not a kohein who went up too late should go down.) Therefore, they should wash their hands before this so they will be ready with clean hands when the shliach tzibbur reaches “R’tzei….” After the fact, if they wash their hands after the shliach tzibbur says “R’tzei…,” it still works.
100:8 When the kohanim go up to duchen, they stand facing the aron (ark), which is (optimally) in the east. (See Mishnah Brurah 128:37 for when the aron is not in the east.) They say Modim d’Rabbanan with the congregation, after which they say, “May it be Your will, Hashem our G-d, that this blessing that You have commanded us to bless Your people Israel will be a perfect blessing and there won’t be any stumbling block or iniquity in it from now until forever.” The kohanim extend this prayer until the shliach tzibbur reaches the end of the bracha so that the congregation replies Amen to this prayer as well. The shliach tzibbur then says, “Our G-d and G-d of our fathers, bless us…” quietly and the word ”kohanim” out loud. With this, he calls upon the kohanim to duchen. He continues “Your holy people” quietly again. After the shliach tzibbur has called out “kohanim,” they begin to recite the bracha that G-d has sanctified them with the holiness of Aaron. They turn around to face the congregation and conclude the bracha that G-d had commanded them to bless Israel with love. The congregation then responds with Amen. The shliach tzibbur, however, does not respond Amen because that would be considered an interruption. Because the bracha recited by the kohanim specifies “with love,” it is understood that if the congregation hates the kohein or vice versa, then he should not bless them (unless he can overcome his hatred – MB 128:37). It is considered a danger to the kohein if he duchens under such conditions and he should leave the shul. If there is only one kohein in shul, the shliach tzibbur does not say “kohanim” out loud (even there are also underage kohanim in shul – MB 128:38). Instead, the lone kohein simply turns around at the appropriate time.