102:1 When yom tov falls on Friday, it is forbidden to bake or cook specifically for Shabbos unless one makes an “eiruv tavshilin” before yom tov. To do this, one takes a cooked or roasted food item that is suitable to eat with bread; he also takes some bread and he recites the bracha that G-d commanded us regarding the mitzvah of eiruv. He then recites a sentence to the effect that by virtue of the eiruv, we may bake, cook, keep food warm, light candles, and do all necessary preparations on yom tov for Shabbos. If one doesn’t understand the Aramaic text, he may recite it in a language he does understand.
102:2 The cooked item used for the eiruv tavshilin must be something that is normally eaten with bread, such as meat, fish or eggs; something that is not normally eaten with bread may not be used. The minimum volume of the cooked item is the size of an olive (a k’zayis – about 31 grams) and the bread must be egg-sized (a k’beitzah – 62 grams). One should try to use the best possible food in honor of the mitzvah. The bread should be a whole roll, it should be placed on the Shabbos table for use as the second loaf, and it should be used for HaMotzi at the third Shabbos meal. The reason is that since it was already used for a mitzvah, it should be use again for another mitzvah.