Since Shabbat is right after Rosh Hashana and one cannot prepare from Yom Tov (Holidays) to Shabbat the sages instituted an ‘Eruv’ in which we prepare food before the holiday for Shabbat so as any further Shabbat preparations that take place on the holiday are only a continuation of the original preparation and are thereby permitted.
This Eruv must be made before Rosh Hashana begins with a baked food, (27 grams of bread) and a cooked food like a hard-boiled egg which should be consumed during the Shabbat.
When setting aside the eruv tavshilin, one should recite this blessing: “Blessed art thou, Lord our God, King of the universe, who sanctified us with his commandments, and commanded us concerning the precept of eruv.”
He should then add: “By virtue of this eruv may I be permitted to bake and cook, cover foods and light candles on the festival day tomorrow what we need for the Sabbath for us and all the inhabitants of this city.”
If he also acts for others, he should say: “May I and so-and-so be permitted,” or: “May all the inhabitants of the town be permitted to bake and cook on the festival for the Sabbath following it.”