The eiruv text

Once the eiruv foods are designated, the proper method of arranging the eiruv is as follows: A brochah is recited, followed by a text stating that through this eiruv it will be permissible to cook, bake, insulate foods, light candles and perform any necessary preparations on Yom Tov for Shabbos. If one is arranging the eiruv on behalf of others, he must say so. According to some opinions, the text should also state that the eiruv will allow for carrying into the reshus harabim. The text may be recited in any language, as long as it is understood by the one who is reciting it.

[שו"ע תקכז, יא-יב, ומשנ"ב לז-מ; ביאורים ומוספים דרשו, 48 ,וראה שם, 49]

Appointing an agent to arrange the eiruv

The poskim disagree about whether an eiruv is valid if the text was omitted. Some poskim argue that it is, since the primary idea of the eiruv is to set aside the foods, not to recite a text. Nonetheless, these opinions agree that the recitation is also a part of the mitzvah. Therefore, the brochah--which is generally made before a mitzvah is done--can still be made after the foods are set aside, since the text was not yet recited. When an eiruv is being arranged by an agent who is using food set aside by the beneficiary, the agent should not recite a brochah; he is merely reciting the text, which is not a critical part of the eiruv.

[משנ"ב תקכז, לו ו־סג, וביה"ל יב, ד"ה ומברך; וראה משנ"ב לז; ביאורים ומוספים דרשו, 52-51] 

Cooking on the first day of Yom Tov for Shabbos

An eiruv only allows a person to cook and bake on Friday. It is forbidden to do any melacha for Shabbos on a Thursday which is the first day of a Yom Tov. This rule applies even if one expressly intended (when arranging the eiruv) to use the eiruv on Thursday, knowing that he may not have time on Friday. This rule certainly applies nowadays, when it is understood that the second day of Yom Tov is in fact a weekday. It is clearly forbidden to cook on Yom Tov for Shabbos when there is a weekday in between.

[שו"ע תקכז, יג, משנ"ב מא, ושעה"צ נח]