- Peninei Halakha
Before counting the omer, one recites the following berakha: "Barukh ata Hashem Elokeinu Melekh ha-olam, asher kideshanu be-mitzvotav ve-tzivanu al sefirat ha-omer ("Blessed are you, Lord our God, King of the universe, Who has sanctified us with His mitzvot and commanded us concerning the counting of the omer"). Both the berakha and the count are optimally recited while standing, but if one recited them while seated, he has nonetheless fulfilled his obligation (sa 489:1).
There are two components to the count: the days and the weeks, as the Torah says:
And from the day on which you bring the omer of elevation offering – the day after the "Shabbat" – you shall count off seven weeks. They must be complete: You must count until the day after the seventh week – fifty days. (Vayikra 23:15-16)
Therefore, one must mention the tally of both days and weeks when counting the omer (Menaĥot 61a). For example, on the seventh day one says: "Ha-yom shiva yamim, she-hem shavu’a eĥad" ("Today is day seven, making one week"). On the fourteenth day, one says: "Ha-yom arba’ah asar yom, she-hem shnei shavu’ot" ("Today is day fourteen, making two weeks"). We mention the number of days and weeks even in the middle of a week. For example, on the tenth day one says: "Ha-yom asara yamim, she-hem shavu’a eĥad ve-shlosha yamim" ("Today is day ten, making one week and three days").
There are several versions of the formula for sefirat ha-omer. Some say la-omer (lit. "to the omer") while others say ba-omer (lit. "of the omer"). Some say it in the middle of the formula ("Today is day fourteen of/to the omer, making two weeks"), while some say it at the end ("Today is day fourteen, making two weeks, of/to the omer." One fulfills his obligation no matter which version he uses. It is customary to add the Le-shem Yiĥud ("For the sake of the unification") paragraph before counting, as well as various other prayers afterward, but one is not obligated to do so. The primary observance is the actual counting and the preceding berakha.
The number seven indicates the complete manifestation of something, as the world was created in seven days. Indeed, every physical entity has six sides – four lateral sides, a top, and a bottom – as well as a seventh aspect: its essence. Man has seven facets as well, which is why it takes seven days to go from a state of defilement to a state of purity. For seven days, one prepares every facet of selfhood to rise from the defilement to purity.
The same is true of purification for sacred endeavors in this world, like eating teruma and sacrificial foods and a woman’s purification for her husband. However, receiving the divine Torah, whose lofty status belongs to the supernal worlds, requires a much deeper count: seven weeks instead of seven days. In this count, each of the seven numbers is manifested through all seven of its facets. Thus, our purification to receive the Torah is complete. Every aspect of our character undergoes refinement and expresses its yearning and anticipation for receiving the Torah.
 The Rishonim find an indirect support for this law in the verse, "From when the sickle is first put to the standing grain (ba-kama)" (Devarim 16:9). The Rishonim explain: Do not read the word as ba-kama but as ba-koma, meaning "while standing." Sefer Ha-eshkol (Hilkhot Pesaĥ 159:1) states that we do not recite the berakha of She-heĥeyanu on sefirat ha-omer because we count in anticipation of Shavu’ot, and the berakha of She-heĥeyanu recited on Shavu’ot covers sefirat ha-omer as well. Maharil suggests that it is because sefirat ha-omer is only a preliminary mitzva that culminates in Shavu’ot. Radbaz 4:256, Maharsham 1:213, and Rav Pe’alim oĥ 3:32 offer these reasons as well. Maharil adds that we are concerned that one may forget to count one day and forfeit the entire count, rendering his initial recitation of She-heĥeyanu a berakha in vain. Kol Bo §145 explains that we omit the berakha because the mitzva is only rabbinic nowadays. Rashba writes (Responsa Rashba 1:126) that one only recites She-heĥeyanu if the mitzva gives one pleasure. We take the lulav to express joy; we blow the shofar as a remembrance; but sefirat ha-omer is merely a preparatory act, which provides no pleasure. Furthermore, sefirat ha-omer today has taken on an additional role as a mournful commemoration of the Temple’s destruction. Rabbeinu Yeruĥam provides the same reason, quoting Ha-ma’or.
 At the completion of every week, one must mention the number of days and weeks, e.g., "Today is day seven, making one week." There is a dispute, however, regarding the middle of each week, such as on the eighth day. According to Ha-ma’or and other Rishonim, one need count only the days and therefore say, "Today is day eight." R. Ephraim maintains that one must count only the weeks and say, "Today is week one, day one." According to Rif, Rambam, and Rosh, one tallies both days and weeks every day, and that is our practice, as sa 489:1 states.
Be-di’avad, if one forgot to count the days on a day that completes a week, he has not fulfilled his obligation, and he must count again with a berakha. If he forgot to rectify his mistake that day, he counts each day from then on without a berakha. If one forgot to count the weeks on a day that completes a week, some maintain that he has fulfilled his obligation be-di’avad, while others maintain that he has not. In the middle of a week, however, such as on the eighth day, if one says merely, "Today is day eight," he has fulfilled his obligation be-di’avad. If he says only, "Today is week one, day one," some maintain that he has fulfilled his obligation. In all of these scenarios, one should count again properly without a berakha. However, if one failed to correct his error on that day, he may count the next day with a berakha (based on mb 489:7, sht ad loc. 9, 19, and section 8 below).