The Halacha of "Tosefot Shabbat" (adding onto Shabbat) requires extending Shabbat on either end, both before sundown on Friday, and after dark on Saturday evening. Therefore, a person must accept upon himself the Shabbat prohibitions several minutes before sundown on Friday afternoon, rather than waiting for these laws to take effect automatically at sundown.
Practically speaking, this means that one must accept Shabbat immediately after reciting Minha in the synagogue on Friday afternoon. One cannot accept Shabbat before reciting Minha, because Minha is a weekday prayer, and thus it must be recited before the onset of Shabbat. Therefore, immediately after reciting the silent Amida of Minha, when a few minutes still remain until sundown, one should accept upon himself the Shabbat prohibitions. According to Halacha, one does not have to verbally declare his acceptance of Shabbat; mental acceptance suffices to fulfill the requirement of Tosefet Shabbat. According to Kabbala, however, it is proper to verbalize one's acceptance of Shabbat, and therefore upon completing the Amida of Minha on Shabbat afternoon, one should declare "Bo'i Kala" ("Come, O bride!") to verbally accept Shabbat and thereby fulfill the obligation of Tosefet Shabbat.
Summary: Halacha requires one to accept the Shabbat prohibitions several minutes before sundown on Friday afternoon. Therefore, after reciting the Amida of Minha before Shabbat, one should declare "Bo'i Kala" and thereby accept upon himself the onset of Shabbat.
