75:3 The best way to fulfill the mitzvah of lighting Shabbos candles is with olive oil (because it draws into the wick very well – Mishnah Brurah 264:23). Sunflower oil is also acceptable, but not every type of oil necessarily is. The wick should be made of cotton, flax, or hemp; again, not every other textile is necessarily usable. Candles made of tallow may also be used but one may not put tallow in a dish with a wick and use it for Shabbos candles. When lighting, one must light until the flame covers the majority of the wick's external portion.
75:4 The general rule when it comes to mitzvos is that one recites the bracha before performing the action. When it comes to lighting Shabbos candles, however, the woman accepts Shabbos upon herself through lighting. The bracha is considered the start of the lighting so if she said the bracha first, she would be accepting Shabbos and then be unable to light. For this reason, she deviates from standard operating procedure and lights the candles before she recites the bracha. In order to make it as if the bracha precedes the action, the woman covers her face with her hands so that she doesn't see the candles. She then recites the bracha, removes her hands from her eyes, and looks at the candles. This is considered as if she recited the bracha before lighting (as she does not benefit from the light until after she has recited the bracha – Rema 263:5). Even though one may light candles on yom tov, we light in this way then as well, in order not to act differently on Shabbos and yom tov. (Authorities differ in this matter – MB 263:27.) While one may light many candles throughout the house for Shabbos, the main ones are the ones on which the bracha was recited; these are the ones that should be on the table where the Shabbos meal is eaten. Accordingly, these candles should be lit last. In a case of great need, such as if a woman must go to the mikvah or to a wedding, or if she must attend to some other pressing matter, she may then stipulate that she does not accept Shabbos by lighting; she may then recite the bracha before lighting. This condition need not be stated aloud.