There is a mitzvah (some say mid’oraiysa) to accept Shabbos some time before sheki’a. One must therefore be careful to daven Mincha on Friday early enough that there will be time to accept Shabbos before sheki’a. Women must daven Mincha before lighting Shabbos candles.
One accepts Shabbos early preferably verbally. However, even if one only thinks in his mind that he accepts Shabbos, it is binding and he has fulfilled the mitzva.