The primary section of the Selichot service is the “יג מדות” the recitation of G-d’s Thirteen Attributes of Mercy, which was established by the Anshei Keneset Ha’gedolah (Men of the Great Assembly). The Kaf Ha’Chaim (a”h) writes in his sefer that it is forbidden to recite the 13 Midot without concentrating on the meaning of the words. The words must be recited with fear, awe and trepidation, and with as intense concentration as possible.
One should bow when reciting the words “Va’yaavor Hashem al Panav vayikra,” and then stand straight for the recitation of “Hashem, Hashem.” The Ben Ish Chai (a”h) states that when reciting “Hashem, Hashem,” one should lift himself with his feet, as we do when reciting “Kadosh, Kadosh Kadosh,” in Nakdishach.
One must recite the 13 Midot in a loud voice, just as Moshe Rabeinu (a”h) declared them in a loud voice.
There is a debate among the mefarshim in identifying the thirteen attributes. We accept the view of the Ari Ha’kadosh (a”h), who listed the attributes as follows: 1) Kel 2) Rachum 3) Chanun 4) Erech 5) Apayim 6) Rav Chesed 7) Emet 8) Notzer Chesed 9) La’alafim 10) Noseh Avon 11) Va’fesha 12) Ve’chata’a 13) Ve’nakeh.
The 13 Midot should be recited together with a minyan. If one is in the middle of the recitation when the congregation completes it, he may nevertheless continue and complete the recitation, since he had begun to recite the Midot together with them. This is the ruling of the Ben Ish Chai.