It is preferable to use red wine, as opposed to white wine, for the four cups of wine at the Seder. The custom of the Sepharadim is to afford preference to red wine even over white wine that people deem superior. Furthermore, one should preferably use wine that is not "Mevushal," meaning, that had not been brought to a boil. Nevertheless, one who drinks "Mevushal" wine fulfills his obligation of the four cups.
One must drink four cups of wine even if this entails difficulty. The Talmud tells of some Rabbis who had to wrap their heads in bandages for seven weeks after the Seder because of the four cups of wine they drank, demonstrating that one must make an effort to fulfill this Misva even if he finds it difficult. Nevertheless, one need not and indeed should not drink wine if this would endanger his health, such as in the case of diabetics or those with other medical conditions that are exacerbated by the ingestion of wine. People who fall under this category should not risk their health for the purpose of drinking the four cups of wine at the Seder.
If a person had taken a vow (a "Neder" or "Shevu'a") to refrain from wine such as a recovering alcoholic who uttered this kind of vow as part of his recovery process must have the vow annulled ("Hatarat Nedarim") in order to be able to drink the four cups at the Seder.
According to Kabbalistic tradition, one must rinse the exterior and interior of the cup before each of the four times he drinks. Furthermore, one should add three drops of water into the wine before reciting the Beracha. A person should ensure not to add more water, as it may dilute the wine and undermine its Halachic status as "wine."
The cup should be at least the size of a "Revi'it," or 3.2 ounces, and one should preferably drink the entire cup. If one finds this difficult, he should drink at least the majority of the cup. Therefore, it is advisable to use a cup that is not much larger than 3.2 ounces, so that one will not have difficulty drinking the entire cup, or at least the majority of the cup. The wine should be drunk without any interruption. One who finds this difficult may sip the wine at his own pace, provided that he drinks the minimum required quantity within four minutes.
When the Seder is held on Mosa'e Shabbat, one recites a special Kiddush known by the acronym "Yaknehaz," which stands for "Yayin" (the Beracha over wine), "Kiddush" (the Beracha recited in honor of the holiday), "Ner" (the Beracha recited over a candle), "Havdala" (the Beracha recited on every Mosa'e Shabbat) and "Zeman" (the Beracha of She'he'heyanu).
The Beracha over the candle "Bore Me'ore Ha'esh" must be recited over an existing flame, or a flame lit from an existing flame; it is forbidden to kindle a new fire on Yom Tov. (It is also forbidden to extinguish a flame on Yom Tov. Therefore, after candle lighting one should let the match extinguish on its own, or give it to a gentile.)
When reciting the Beracha of "Havdala" in this case, one must remember to conclude with the words "Ha'mavdil Ben Kodesh Le'kodesh," as opposed to the usual text of "Ha'mavdil Ben Kodesh Le'hol." Nevertheless, if one mistakenly recited "Ha'mavdil Ben Kodesh Le'hol," he has fulfilled his obligation and need not recite the Beracha again. Furthermore, one should ensure to pause after reciting the words "Ve'et Yom Ha'shevi'i Mi'sheshet Yeme Ha'ma'aseh Kidashta" before the words "Ve'hivdalda Ve'kidashta Et Amecha Yisrael." This applies as well in the paragraph of "Va'to'di'enu" which is added to the Arvit service when Mosa'e Shabbat coincides with Yom Tov.
If a person forgot to recite Havdala during Kiddush, and he remembers before he began the Maggid section of the Seder, he should immediately recite the Beracha of Havdala that he mistakenly omitted. If he remembered only after he began Maggid, then he recites the Beracha of Havdala together with the Beracha of "Ga'al Yisrael" recited over the second cup of wine.
If one forgot to recite the Beracha over the candle during Kiddush, then he recites it as soon as he remembers, even during the Maggid section, as one should not derive benefit from light before reciting this Beracha.