The Laws of Prayer (The morning services) (37)

1. When a person knows that he is going to say one of the prayers which are uncommon, like the prayers of the festivals and the new month, and “Al Hanissim” and the blessing on the new moon, he must first prepare the wording of the prayer and only pray afterwards, so that his prayer is fluent in his mouth. However, if he prays from a prayer-book, he does not need to prepare it beforehand. (Shulchan Aruch and Mishne Berurah 98)

2. On the other hand, those prayers which are difficult to understand, like those of the High Holidays, even when praying from a prayer-book, one must prepare them before the commencement of the prayers. (ibid)