Siman 565
Presented here for study, not as a practical ruling. The Shulchan Aruch rules by Sephardi custom, the Rema's glosses by Ashkenazi custom, and the Kitzur follows Ashkenazi custom. For a practical question, ask your rabbi.
א יָחִיד אוֹמֵר עֲנֵנוּ בְּשׁוֹמֵעַ תְּפִלָּה, בֵּין יָחִיד שֶׁקִּבֵּל עָלָיו תַּעֲנִית בֵּין יָחִיד הַמִּתְפַּלֵּל עִם הַצִּבּוּר בְּתַעֲנִית צִבּוּר; וְכֵן שְׁלִיחַ צִבּוּר שֶׁמְּסַדֵּר תְּפִלָּתוֹ בְּלַחַשׁ, כְּיָחִיד דָּמִי וּבְשׁוֹמֵעַ תְּפִלָּה. הַגָּה: וְאוֹמְרוֹ קֹדֶם כִּי אַתָּה שׁוֹמֵעַ וְכו', וְלֹא יַחְתֹּם בְּעוֹנֶה בְּעֵת צָרָה, אֶלָּא כְּשֶׁמַּגִּיעַ לְכָל עֵת צָרָה וְצוּקָה יְסַיֵּם כִּי אַתָּה שׁוֹמֵעַ תְּפִלַּת וְכו' (בֵּית יוֹסֵף בְּשֵׁם תּוֹסָפוֹת); וְלֹא יְשַׁנֶּה מִמַּטְבֵּעַ בְּרָכָה. אֲפִלּוּ כְּשֶׁמִּתְעַנֶּה יָחִיד, יֹאמַר: עֲנֵנוּ בְּצוֹם תַּעֲנִיתֵנוּ וְכו'. (רַשְׁבָּ"א סִימָן כ"ה).
1 The individual says "Anenu" in the "Shomea Tefila" [blessing of the Amidah]. This is true both of an individual who has accepted an individual fast and an individual who is praying communally during a communal fast. The prayer leader who prays silently is the same as an individual in this respect and says it in "Shomea Tefila." Rema: He says it prior to "Because you hear..." He does not finish the prayer with "[Blessed are You, God] who listens during a time of trouble. Rather, when he gets to "for every period of trouble and strife," he finishes "because you hear..." (Beit Yosef in the name of the Tosafot). He should not change the form of the blessing. Even a person fasting individually should say "Respond to us on our fast day" (Rashb"a Siman 55).
ב אִם שָׁכַח מִלּוֹמַר עֲנֵנוּ, אֵין מַחֲזִירִין אוֹתוֹ וְעַיֵּן לְעֵיל סִימָן רצ"ד סָעִיף ד' וְה'; וְאִם נִזְכַּר קֹדֶם שֶׁעָקַר רַגְלָיו, אוֹמֵר בְּלֹא חֲתִימָה לְאַחַר תְּפִלָּתוֹ.
2 If one forgot to say "Anenu," he does not repeat the amidah (see above in siman 244 section 4-5). If he remembers before moving his feet, he should say it without the final blessing ''(after the end of his Amidah).''
ג יֵשׁ אוֹמְרִים שֶׁאֵין הַיָּחִיד אוֹמֵר עֲנֵנוּ, כִּי אִם בְּמִנְחָה, שֶׁמָּא יֹאחֲזֶנּוּ בֻּלְמוֹס וְנִמְצָא שַׁקְרָן בִּתְפִלָּתוֹ; אֲבָל שְׁלִיחַ צִבּוּר, אוֹמְרוֹ גַּם בִּתְפִלַּת שַׁחֲרִית כְּשֶׁהוּא מִתְפַּלֵּל בְּקוֹל רָם, שֶׁאִי אֶפְשָׁר שֶׁלֹּא יִתְעַנּוּ קְצָת מֵהַקָּהָל; וּבְאַרְבַּע צוֹמוֹת, גַּם הַיָּחִיד אוֹמְרוֹ בְּכָל תְּפִלּוֹתָיו, דַּאֲפִלּוּ יֹאחֲזֶנּוּ בֻּלְמוֹס וְיֹאכַל שַׁיָּךְ שַׁפִּיר לְמֵימַר עֲנֵנוּ בְּיוֹם צוֹם הַתַּעֲנִית הַזֶּה, כֵּיוָן שֶׁתִּקְּנוּ חֲכָמִים לְהִתְעַנּוֹת בּוֹ. הַגָּה: וְנָהֲגוּ בְּכָל הַצּוֹמוֹת שֶׁלֹּא לְאָמְרוֹ כִּי אִם בְּמִנְחָה, מִלְּבַד שְׁלִיחַ צִבּוּר שֶׁאוֹמֵר שַׁחֲרִית כְּשֶׁמִּתְפַּלֵּל בְּקוֹל רָם.
3 There are some who say that the individual only says "Anenu" in the afternoon prayer, lest he become faint [and need to break the fast] and it turn out that his prayer was a lie. However, the prayer leader should say it in the morning prayers during the repetition, because it is impossible that part of the congregation will not fast. On the four [official communal] fasts the individual should also say it in all of his prayers, because even if he becomes faint and eats, it still will make sense to say "Answer us on this fast day" because the Sages decreed a fast. Rema: The practice on all fasts is to only say it in the afternoon, except for the prayer leader who says it during the morning repetition.
ד טוֹב לוֹמַר בַּתַּחֲנוּנִים שֶׁאַחַר תְּפִלַּת הַמִּנְחָה לְאַחַר אֱלֹהַי נְצֹר וְכו': רִבּוֹן כָּל הָעוֹלָמִים, גָּלוּי וְיָדוּעַ לְפָנֶיךָ שֶׁבִּזְמַן שֶׁבֵּית הַמִּקְדָּשׁ קַיָּם אָדָם חוֹטֵא וּמַקְרִיב קָרְבָּן וְאֵין מַקְרִיבִים מִמֶּנּוּ אֶלָּא חֶלְבּוֹ וְדָמוֹ וְכו'.
4 It is good to say the supplications after the afternoon prayer that are after "God, guard...": "Master of the Universe, it is revealed and known to You that in the time that the Temple existed, a person would sin and bring a sacrifice, and he would only bring its suet and blood..."
ה אֵין הַיָּחִיד רַשַּׁאי לוֹמַר שְׁלֹשׁ עֶשְׂרֵה מִדּוֹת דֶּרֶךְ תְּפִלָּה וּבַקָּשַׁת רַחֲמִים, דְּדָבָר שֶׁבַּקְּדֻשָּׁה הֵם; אֲבָל אִם בָּא לְאָמְרָם דֶּרֶךְ קְרִיאָה בְּעָלְמָא, אוֹמְרָם. הַגָּה: וְכֵן אֵין לְיָחִיד לוֹמַר סְלִיחוֹת אוֹ וַיַּעֲבֹר (מַהֲרִי"ל בְּשֵׁם אוֹר זָרוּעַ).
5 The individual may not say the Thirteen Attributes in a prayerful manner, requesting mercy, because they are words of holiness [that require a minyan]. However, if he says them by merely reading them, he may say them. Rema: Similarly, the individual may not say Selichot or "And God passed..." (Mahari"l in the name of the Or Zarua).
ו הַמִּתְעַנֶּה וּמְפַרְסֵם עַצְמוֹ לַאֲחֵרִים לְהִשְׁתַּבֵּחַ שֶׁהוּא מִתְעַנֶּה, הוּא נֶעֱנָשׁ עַל כָּךְ.
6 One who fasts and publicizes himself to brag that he is fasting is punished.