Halachos Of The Seder:
1. Although in general, one may recite Kiddush and begin eating one's meal before nightfall on Shabbos and Yom Tov, and thus add from "non-holy time" to "holy time" (1), this is not allowed on Pesach.
[The rationale is] that the mitzvah of eating matzah must be fulfilled at night, [at the time] when the Pesach sacrifice [was to be eaten], as [Exodus 12:8 states]: "And they shall eat the meat on this night." Similarly, the mitzvah of drinking the four cups of wine must be fulfilled at night. Since the cup [of wine] over which Kiddush is recited is one of the these four cups, the Kiddush should not be recited until it is definitely "night" (that is, after the appearance of three medium-size stars).
[Shortly after nightfall], one should put on the kittel and sit at one's place for the Seder. It is a mitzvah to give children almonds, nuts, and the like to play with, so that they will notice that something is different about this night, and ask about it. [Hopefully,] this will motivate them to ask other questions concerning the matzoh, the marror, and the reclining (2).
When a child is old enough to be educated (3), and he appreciates the sanctity of Yom Tov, and understands the narrative of the exodus from Egypt, he should also be given a cup [of grape juice, or wine] to drink from (4). It is customary to pour an additional cup of wine, besides the cup given to each of the Seder participants. This is called Elijah's cup. (5)
(1) This concept is called "Le'hosif Mechol al Hakodesh," which generally means to "bring in" Shabbos or Yom Tov before nightfall; once one has consciously accepted Shabbos or Yom Tov, all their laws and restrictions apply.
(2) It is a mitzvah to do something at the start of the seder that is not done on other nights of the year, in order to stimulate the children's curiosity and draw them into asking why this night is different from all other nights - "Ma Nishtana Halaila HaZeh..." Asking the "Four Questions" during the seder is not supposed to be a ritual formality; rather, there is an obligation to do things that will stimulate actual curiosity and questioning, because answers do not make a difference to people who don't have questions (See Rambam, Yad, Chometz U'Matzah 7:3).
(3) That is, when he reaches the age of "chinuch," which varies according to the individual child's level of awareness.
(4) It is a mitzvah to have the child perform the mitzvah of the four cups, however it is not an obligation, because many authorities ruled that the mitzvah of the four cups was not ordained for children.
(5) From the wording of the Kitzur Shulchon Oruch, it appears that Elijah's cup should be poured at the beginning of the Seder. This is the custom in many communities. In others, however, Elijah's cup is not poured until after the meal is concluded.
2. One's servant or another member of the household should pour the wine into the cups. Similarly, each time the cups are filled, they should be filled by these individuals and not by the person leading the Seder himself; this is an outward expression of freedom (1).
One should instruct the members of one's household to drink the majority of the cup in one gulp, and to drink at least a revi'is (2) for the fourth cup (3). They must also have the intent to fulfill [four different] mitzvos: the drinking of the four cups of wine, the relating of the story of the exodus from Egypt (4), and eating matzoh and marror. Women are also obligated to fulfill these mitzvos, but it is not customary for them to recline.
One should recite Kiddush as it is printed in the Haggodos and drink the cup while reclining on the left side.
If possible, it is proper to follow the opinion of the authorities who require that one drink the entire cup of wine for each of the four cups (See note 2).
(1) Royalty and nobility never pour their own cup of wine. Each person at the table who is performing the mitzvah of drinking four cups, should have someone else pour the wine each time.
(2) A "revi'is" literally means "one quarter" because it is equal to the volume of one quarter of a "log." There is a dispute among the authorities as to what a revi'is equals in modern measurements. The opinions range from 88.7 ml to 150 ml (5.07 fluid ounces).
If the cup is capable of containing only a revi'is, one should preferably drink the entire cup of wine for each of the four cups. However, if one is unable to complete the cup, one can fulfill one's mitzvah by drinking most of the revi'is, a measure referred to as "maleh lugmav," which literally means a "cheek-full" (a quantity of liquid which fills one cheek of an average person).
If the cup is capable of holding more than a revi'is, the Ramban rules that one must drink most of the contents of the cup, whereas the Ran rules that it is sufficient to drink most of a revi'is. One should preferably conduct oneself in accordance with the Ramban, however, if one only drank most of a revi'is of a large cup (containing more than a revi'is), one has fulfilled his obligation ("Halachos of Pesach," by Rav Shimon Eider Vol II, pg 230 (Ch20 E1))
The preferred manner of drinking the revi'is is by drinking most of the revi'is in one gulp, and completing the revi'is in the second gulp. If it takes him more than nine minutes to drink the revi'is, he must drink the cup again, with a new blessing, in order to fulfill the mitzvah ("Halachos of Pesach," by Rav Shimon Eider Vol II, pg 233 Ch20E3).
One should make sure to drink an entire revi'is for the fourth cup, because the "bracha acharona" ("after-blessing"), recited after the fourth cup, may only be recited after drinking a revi'is.
(3) Even if drinking four cups of wine will cause someone slight discomfort, such as a mild headache, one is still obligated to do so. One may dilute the wine with grape juice or water (preferably grape juice), but one should try to do it in a way that the alcoholic taste of the wine remains. Also, when diluting with water, one must be careful to use the ratio required to retain the blessing of "Borei Pri HaGofen" (more than one part wine to six parts water; the fact that some modern wines are already diluted must be taken into account).
One is not obligated to do anything that will cause him to become bedridden. Therefore, if one is unable to drink any wine (or can only drink a small amount), one may fulfill the mitzvah of the four cups using just grape juice. If one is allergic to grape juice, one may fulfill the obligation by drinking four cups of "Chamar Medina" ("beverage of the country"), which is generally defined as a drink one would serve to a guest even when he is not thirsty, such as alcoholic beverages (make sure they are not chometz), tea, and coffee ("Halachos of Pesach" by Rabbi Shimon Eider, Vol II, pg 220-225; for further details, ask your local orthodox rabbi).
(4) Relating the story of the exodus on the night of Pesach is a Biblical commandment.
3. Afterwards, [each person] should wash his hands without reciting the blessing (1), and then dry them. [The person leading the Seder] should cut a piece of the "karpas" (2) for himself and for all the members of his household. Each person should receive a piece [of the vegetable which is] less than a "k'zayis" in size (Lit: "like an olive") (3).
The pieces are dipped in salt water and the blessing "borey pri ho'adomoh" is recited. While reciting this blessing, one should have in mind that it should also apply to the marror (a vegetable to be eaten later in the evening). One should also eat the "karpas" while reclining on one's left side (4).
Afterwards, one takes the middle matzah and divides it into two portions, placing the larger portion next to him so that it can be used later as the "afikoman". It is customary to wrap the afikoman in a cloth to recall [Exodus 12:34]: "Their left-over dough was wrapped up in their clothes." There are some who place the matzah in the cloth on their shoulders to recall [the manner in which our ancestors] left Egypt.
The afikomon takes the place of the Pesach sacrifice. Therefore, the greater portion of the matzah should be used for it. The smaller portion should be returned to its place on the Seder plate. The matzos should be uncovered slightly and the plate lifted up. We then say the passage beginning "Ho Lachmah Anya... " ("This is the poor man's bread") until its conclusion with "next year we will be free people."
[Certain versions of the Haggodoh read] "K'ho lachmoh anya... " [instead of "Ho lachmah anya..."]. According to these versions, the passage continues, "..acholu avhosono..." [rather than "di acholu avhosono..."].
(1) That is, the blessing for washing one's hands - "Al Netilas Yadayim." There is a halacha that before eating (with one's hands) any food that has been washed or dipped in water (and is still wet), one has to perform the same ritual washing that one would perform before eating bread. However, many authorities rule that this halacha only applied when the Beis Hamikdash (Temple) was standing; nevertheless, even these authorities rule that this washing must be performed on Pesach, as an additional means of arousing the curiosity of the children.
(2) See HY 118:2
(3) Volume of approximately one fluid oz, 28.8 cubic centimeters.
(4) Many authorities do not require one to recline while eating the karpas. Unless one's custom is to the contrary, it is preferable to recline.
4. After reciting the paragraph beginning: "This is the bread of affliction..."], the second cup of wine is poured. A young child should ask [the four questions, beginning] "Mah nishtanah...". If there are no young children present, another son or daughter, or a friend or one's wife should ask the questions (1). After that, [the Haggodoh continues with (2)] "Avodim hoyinu.." ("We were slaves...").
It is proper to explain the Haggodoh to the members of one's household in a language that they understand. If [the person leading the Seder] does not understand Hebrew, he should recite the Haggodoh from a text with translation. After reciting each passage of the Haggodoh [in Hebrew], he should translate it. In particular, this applies to the passage beginning: "Rabban Gamliel would say, " for it is essential for the participants to understand the reasons for [the mitzvos of] the Pesach sacrifice, matzoh, and marror.
When one reaches the passage beginning: "V'hi she'omdoh...", one covers the matzos (so the matzos will not be "shamed" when the cup of wine is lifted) and raises the cup. One should hold it upraised until the word "miyodom" [at which point, it is lowered] and the matzoh uncovered again.
When the passage beginning: "Matzoh zo.." ("This matzah") is reached, the person leading the Seder should lift up the broken piece [of the middle matzoh] from the Seder plate and show it to the members of his household. Similarly, when reciting "Marror zo", one should lift up the marror. When, however, reciting the passage "Pesach shehoyu avoseinu ochlim...", one should not lift up the "zeroa," even though it commemorates the Pesach sacrifice. Doing so would make it appear that this meat was being consecrated for sacrificial purposes (3).
When one reaches the passage beginning: "Lefichoch..." ("Therefore..."), one should cover the matzos and raise the cup until one concludes the blessing "Go'al Yisroel". One then recites the blessing "borey pri hagofen" and drinks this second cup of wine while reclining on one's left side.
(1) If one is alone, one must ask oneself the four questions.
(2) One does not recline while reciting the Haggadah.
(3) Which is impossible when there is no Beis Hamikdosh (Temple) standing in Jerusalem.