You are now ready to perform the mitzvah of affixing the mezuzah. The
blessing is recited once, before putting up the mezuzahs. You will put up the first mezuzah on the most important doorway--your front door, so that's where you should be standing, mezuzah and tools in hand. As you recite the blessing, keep in mind that it applies to all the mezuzahs you will be presently affixing in your home.
Recite the blessing:

Baruch Atah A-do-nai Elo-heinu Melech haolam asher kideshanu bemitzvotav vetzivanu likboa mezuzah.
Blessed are you, G-d our G- d, King of the Universe, Who has made us holy with his mitzvahs and commanded us to affix a mezuzah.