The primary section of the Selihot service is the "Yag Middot," the recitation of G-d’s Thirteen Attributes of Mercy, which was established by the Ansheh Kenesset Ha’gedola (Men of the Great Assembly). The Kaf Ha’haim (Rav Yaakob Haim Sofer, Baghdad-Israel, 1870-1939) writes (581:5; listen to audio recording for precise citation) that it is forbidden to recite the "Yag Middot" without concentrating on the words’ meaning. He adds that 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’yaabor Hashem Al Panav Vayikra," and then stand straight for the recitation of "Hashem Hashem." The custom in Yeshivat Bet-El was to stand up straight when reciting "Vayikra," and then bow again for "Hashem Hashem."
The Ben Ish Hai (Rav Yosef Haim of Baghdad, 1833-1909) writes that when reciting "Hashem Hashem," one should lift himself with his feet, as we do when reciting "Kadosh Kadosh Kadosh" in Nakdishach. One must make a brief pause between the first "Hashem" and the second, as they are separated by a "Pesik" note. The Kaf Ha’haim writes that one who recites them without a brief pause is liable to severe punishment. One should also briefly pause in between the words "Vayikra Be’shem" and "Hashem."
One must recite the "Yag Middot" in a loud voice, just as Moshe Rabbenu declared the attributes in a loud voice.
There is a debate among the commentators in identifying the thirteen attributes. We accept the view of the Arizal (Rav Yishak Luria of Safed, 1534-1572), who listed the attributes as follows: 1) Kel; 2) Rahum; 3) Hanun; 4) Erech; 5) Apayim; 6) Rab Hesed; 7) Emet; 8) Noser Hesed; 9) La’alafim; 10) Noseh Avon; 11) Va’fesha; 12) Ve’hata’a; 13) Ve’nakeh. It is proper to count the attributes with one’s fingers while reciting them.
The "Yag Middot" may only 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 "Yag Middot" together with them. This is the ruling of the Ben Ish Hai (Parashat Ki-Tisa).