Rosh Chodesh order

In the morning and evening prayers, one says "go up and come" with "Retzah"; and if one does not say it in the morning, one does not return it - between one day and two days - because the month is not sanctified at night; but if one does not say it in the morning and evening prayers, one returns it. And if one remembers before he begins "Modim", he says it in the place he remembers; and if one remembers after saying "Blessed are you, O Lord", he ends with "Learn from me your laws". Some say that one does not finish "Learn from me your laws", but rather completes the blessing, and then says "go up and come" before "Modim". And if one does not remember until after he begins "Modim": if one remembers before he has completed his prayer, he returns to "Retzah"; and if one does not remember until he says "Let the words of my mouth be pleasing" and is ready to remove his feet, and does not say any more supplications, then it is considered as if he removed his feet and returned to the head. (Ba'ach Shab 2 Leviticus 15)

The recitation of Hallel is performed standing up and on the first day of Chashod and the last six days of Pesach, there is no end to the recitation of Hallel. On the first day of Pesach (and abroad also on the Monday of Pesach), Shavuot, Sukkot, Shemini Atzeret, and Chanukah, a complete Hallel is recited with a blessing at the beginning and end.