Birkat Kohanim falls under the category of "Debarim She’bi’kdusha" – recitations which require the presence of a Minyan. Hence, the Kohanim recite this blessing only in a Minyan, and thus one who prays alone does not receive the full benefits offered by this special Beracha.
Nevertheless, the Ben Ish Hai (Rav Yosef Haim of Baghdad, 1833-1909), in his work Od Yosef Hai (Parashat Tesaveh, Halacha 1), advises those who cannot pray in a Minyan to add a special prayer at the end of the Amida, before Oseh Shalom, asking Hashem to bestow upon them the blessings of Birkat Kohanim. After the prayer, one should recite twice the final three verses of the Book of Micha ("Mi Kel Kamocha…").
In this context the Ben Ish Hai also notes that one who had a bad dream, or is concerned about a dream that he had, should recite during Birkat Kohanim a special prayer ("Halma Taba Hazai…") which is found in most Siddurim. The Ben Ish Hai writes that one should recite this text three times, and try to conclude this recitation at the time the Kohanim recite the final word of Birkat Kohanim ("Shalom"). This way, the "Amen" recited by the congregation to the Kohanim’s blessing will affect this individual’s prayer, as well, and he will thereby receive protection from any harm foretold by his dream.
Daily Halacha
Reaping Benefits of Birkat Kohanim When Praying Alone
Lectured by Rabbi Eli Mansour