We count Sefiras Ha’Omer at night, from the second night of Passover ntil the night before Shavuos. Each night we count what number that day is, from one until forty-nine. We count the final night, number 49, the night before the Holiday of Shavuos.
We count the Omer while standing. (If you counted while sitting, don’t repeat the blessing.) First we recite the blessing for counting the Omer, then we count the correct count for that night, and then we pray that the Holy Temple Service be returned to us in its proper place soon. Those who follow the Customs of the Arizal also add a preparatory prayer beforehand, as well as several prayers afterward. These prayers are not mandatory, but they are very beneficial, and they certainly help us better understand some of the deeper meanings of Counting the Omer.
One should count the Omer as soon as it is definitely full night according to the Torah. This is not done by visual confirmation, but by calculation. This is a complicated issue, so each community and individual must follow their own Rabbi. It depends on several matters, including the latitude of the area in which you live, and various issues concerning the calculations themselves. Instead of giving a long list of possibilities, I will simply tell you to do as your local Orthodox Synagogue does (if they are following the instructions of a Rabbi).
If you count too early, you have not fulfilled the Mitzvah at all. After the proper time begins, you must once again say the blessing, and count again.
Since you must recite the blessing before you count, don’t mention the count for that night beforehand. So if someone asks you what the count is that night, answer by telling him what last night was. If you accidentally tell him “Tonight is the sixteenth,” you may no longer recite the blessing that night.
If you make a mistake and count the wrong number one night, correct yourself as soon as possible, but do not repeat the blessing. By Law, you must know what number you will count that night before you say the blessing.
Do not say anything else at all between the blessing and the counting. If you say anything not related to the Counting of The Omer, you must repeat the blessing.
If you forget to count when full night begins, you can still count all night. If you forgot until the morning, you may count all that day, but without a blessing. The next night, you continue as usual, saying the blessing and counting, and continue incrementing each day, as usual. If, however, you passed an entire day without counting that day of the Omer, or if you counted the wrong number for that day and passed the entire day without correcting it, you may no longer recite the blessing for counting the Omer, even on subsequent nights.
So if, for example, you counted the Omer properly each night for 14 days, then on the fifteenth night you forgot to count the Omer, or you counted the wrong number, you should recite the number for that day the next morning, or as soon as you remember. If you forgot to count, and did not remember all the next day, and suddenly when you are ready to count the sixteenth night you remember that a whole day has passed without you counting, and that you never counted the fifteenth day, you must count the sixteenth night without a blessing. And not only the sixteenth night, but all the remaining days of that count. For the rest of the Omer that year you count but do not say the blessing. (The other prayers you may still say.) This does not affect your counting subsequent years. (The same applies if you counted the wrong number one night and did not correct it all day.)
If in the morning you are not sure if you counted last night, assume you did not, and count without saying the blessing.
If one night you are about to count the Omer, and suddenly you realize that you are not sure if you counted last night, assume you did, and say the blessing before counting.
If you are about to count, and you realize that you don’t know what number to count, don’t count. Wait until you find out what the correct number is for that day, even if you have to miss it the entire day. If you think you know what day it is but you are not sure, count without a blessing. If you find out the next day that you counted correctly, then subsequent nights you may say the blessing before counting.
The Torah tells us to count the weeks as well as the days. Therefore, once a week has passed, we mention the days and the weeks, and the days in the weeks. For example, on the eighth night of the Omer, we count “Today there are eight days, which is one week and one day, of the Omer.” On the thirtieth night, we count “Today there are thirty days, which is for weeks and two days, of the Omer.” And so on.
If one night you counted the day and forgot to count the weeks, you may still say the blessing before counting on subsequent nights.
It is forbidden to eat anything, or get involved in any work, before counting the Omer, for fear that we might forget to count that night.
If you do not understand what the words of the count mean, you should count in a language you understand.
For the prayers, blessing, and the count itself, the best approach is to buy a prayerbook called the Artscroll Siddur, and recite as printed there. You can buy a copy at Tiferes Stam.