Mussaf Shabbos Rosh Chodesh has as its central beracha the special beracha which starts with the words “attah yatzarta.”
- If a person said the right beracha, but concluded wrongly (i.e. he said “mekadesh haShabbos”)
- If the person remembered immediately (within a second), he/she should immediately add “veyisrael v’roshei chodoshim.”
- If the person remembered after a second, he/she should just continue.
- If the person said the regular Shabbos beracha (i.e. he said “Yismach Moshe”)
- If the person remembered before saying the shem HaShem at the conclusion of the beracha, he should stop where he is and start from “attah yatzarta.”
- If the person remembered after concluding the beracha, but before starting retzei, he/she should say ״ונעשה לפניך קרבן ראש החודש הזה״ and then just continue.
There are more rules, but we will stop here.
You should daven from a siddur and remember that there’s a special beracha to be said as the central beracha.