Siman 100
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 תְּפִלּוֹת שֶׁל מוֹעֲדוֹת וְשֶׁל רֹאשׁ חֹדֶשׁ, צָרִיךְ לְהַסְדִּיר תְּפִלָּתוֹ קֹדֶם שֶׁיִּתְפַּלֵּל, כְּדֵי שֶׁתְּהֵא שְׁגוּרָה בְּפִיו. הַגָּה: וְיֵשׁ אוֹמְרִים דַּוְקָא כְּשֶׁמִּתְפַּלְּלִים עַל פֶּה, אֲבָל כְּשֶׁמִּתְפַּלְּלִין מִתּוֹךְ הַסִּדּוּר, מֻתָּר, דְּהָא רוֹאֶה מַה שֶּׁמִּתְפַּלֵּל וְכֵן נוֹהֲגִין, (בֵּית יוֹסֵף בְּשֵׁם הר"ר מָנוֹחַ).
1 The Prayer [i.e. Amidah] of a Festival Needs to be Organized, containing 1 se'if: [Regarding] prayers [i.e the Amidah] of the festivals and of the new month, one needs to organize one's prayer before one prays so that it will be fluent on his tongue. Rema: And there are those who say that [this is] specifically when we pray by heart, but when we are praying from within a prayer book, it is permitted [to pray without organizing] since one sees what one is praying, and this is what we practice. (Bet Yosef in the name of Rav R. Manoah, and so too wrote Tanya in the name of R.A.