Halakhah for Menachot 131:18

From the laws of the commandment is that which they, may their memory be blessed, said (Menachot 66a) that it is a commandment to tally them from the evening so that they be complete. As the verse stated, "complete shall they be" - and they, may their memory be blessed, said, "From when are they complete? From when he begins from the evening." And nonetheless, the commentators (Tosafot in the name of Behag on Menachot, s.v. zecher) explained that if he forgot and did not tally from the evening, he [may] tally on the morrow the whole day. And some say there that one who forgot and did not tally one day may not tally again that year, since they are all one commandment; and since he forgot one day from them, the entire count is negated for him. And our teacher in our generation did not concede to this reasoning. Rather, one who forgot a day should say, "Yesterday was such," without a blessing; and tally the other [days] with all of Israel. And it is a choice [fulfillment] of the commandment to tally [while] standing (Mishneh Torah, Laws of Daily Offerings and Additional Offerings 7:23). And he recites the blessing, "who has sanctified us, etc." And one who has tallied without a blessing has fulfilled [the commandment], and [hence] he is not permitted to return and tally with a blessing (Mishneh Torah, Laws of Daily Offerings and Additional Offerings 7:25). And the rest of its details are in Tractate Menachot (see Tur, Orach Chaim 479).