Different customs exist regarding the procedure for the weekly Friday night candle lighting. Some Sephardic women follow the custom of the Ashkenazim to first light the candles before reciting the Beracha "Le’hadlik Ner Shel Shabbat." This was the practice advocated by the Ben Ish Hai (Rav Yosef Haim of Baghdad, 1833-1909). Although we generally recite the Beracha before performing the Misva, in the case of Shabbat candle lighting the candles are lit before the Beracha – according to this custom – because the recitation of the Beracha "Le’hadlik Ner Shel Shabbat" may constitute the acceptance of the onset of Shabbat. Once the woman accepts the onset of Shabbat, of course, she may not light candles. Therefore, according to this view, the candles must be lit before the recitation of the Beracha.
Others, however, recite the Beracha before lighting the candles, and have in mind not to accept the onset of Yom Tob until after the lighting.
This discussion applies only to the Friday night candle lighting. When it comes to candle lighting for Yom Tob, all women should first recite the Beracha and then light the candles, regardless of their custom on Friday night. Unlike on Shabbat, on Yom Tob it is permissible to light a candle from an existing flame, and thus even if the recitation of the Beracha indeed marks the onset of Shabbat, it is still permissible to light the candles afterward. Therefore, on Yom Tob, even women who on Friday night light the candles before reciting the Beracha should recite the Beracha "Le’hadlik Ner Shel Yom Tob" before lighting the candles. This was the position of Hacham Bension Abba Shaul (Israel, 1923-1998), and this is the ruling of Hacham Ovadia Yosef (Hazon Ovadia – Hilchot Yom Tob, p. 305).
The Magen Abraham (Rabbi Abraham Gombiner, Poland, 1635-1682), in Siman 263, applied to this issue the concept of "Lo Pelug," that a uniform procedure should be followed, and thus even when lighting Yom Tob candles one should first light before reciting the Beracha. Halacha, however, does not follow this view, and thus all women should first recite the Beracha before lighting the Yom Tob candles.
Incidentally, regarding the Friday night candle lighting, Hacham Baruch Ben-Haim instructed "Al Titosh Torat Imecha" – that every woman should follow the procedure which her mother followed, and should not deviate from her mother’s practice. If a woman does not have an established custom from her mother, then she should first recite the Beracha having in mind not to accept Shabbat, and then light the candles.
Summary: On Friday night, some women have the custom to recite the Beracha before lighting the candles, while some light and then recite the Beracha, and every woman should follow her mother’s practice in this regard. When lighting Yom Tob candles, however, all women should first recite the Beracha and then light the candles.
