The berachah for all mitzvos must be recited before doing the mitzvah. This rule, however, does not apply to the lighting of the Shabbos candles. By lighting the candles, the woman thereby ushers in the Shabbos. Since the recitation of the berachah is considered to be the beginning of the act of candle lighting, she has thereby ushered in the Shabbos and may no longer light the candles. Therefore, she should first light the candles, and in order that the berachah precede the actual mitzvah, she should cover her face with her hands in order not to see the candles, then recite the berachah, remove her hands and look at the candles. In this manner, it is as if she has recited the berachah before lighting the candles.
It is customary for the husband to prepare the candles and wicks before Shabbos.
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
לְהַדְלִיק נֵר שֶׁל שַׁבָּת: to kindle the Shabbos light.