On Shabbos and Yom Tov, you are forbidden to go more than 2,000 amos from the place you were at the onset of Shabbos. If you need to walk farther than the Shabbos techum or boundary, you must place an eiruvei techumin (a boundary eiruv) before Shabbos or Yom Tov.
The procedure is as follows. You should take enough bread for two meals and go to the place where the eiruv is to be placed (within the 2,000 amos) and recite the following berachah:
בָּרוּךְ אַתָּה יְהֹוָה Blessed are You, Adonoy
אֱלֹהֵֽינוּ מֶֽלֶךְ הָעוֹלָם our God, King of the Universe,
אֲשֶׁר קִדְּ֒שָֽׁנוּ בְּמִצְוֹתָיו Who sanctified us with His commandments
וְצִוָּֽנוּ and commanded us
עַל מִצְוַת עֵירוּב: concerning the mitzvah of eiruv.
בַּהֲדֵין עֵירוּבָא By means of this eiruv
יְהֵא שָׁרָא לָנָא it will be permissible for me
לֵילֵךְ מִמָקוֹם זֶה אַלְפַּיִם אַמָּה to walk from this place two thousand amos
לְכָל רוּחַ: in each direction.”
You may place one eiruv for many Shabbosos, but you should put it in a safe place, so that it will not be lost or spoiled.