One can accept tosefet Shabbat verbally by saying “I hereby accept upon myself the sanctity of Shabbat.” Some maintain that one can even accept tosefet Shabbat mentally (MB 261:21). Once one has accepted Shabbat, he must be careful not to do any melakha (as will be explained in the next section).
As mentioned, women customarily accept Shabbat when reciting the berakha over lighting Shabbat candles. Since they invoke Shabbat, they intend to accept it upon themselves, thus fulfilling the mitzva of tosefet Shabbat. According to most authorities, a woman may light candles conditionally, having in mind that she is not accepting Shabbat with the lighting. In such a case she may do melakha or drive to shul after lighting. Nevertheless, it is preferable that she accept Shabbat when she lights, since some maintain that one may not light conditionally, in which case once she lights Shabbat candles she has accepted Shabbat and may not do melakha. Furthermore, if she does not accept Shabbat when she lights candles, there is concern that she will forget tosefet Shabbat altogether (SA 263:10; SSK 43:24; Yalkut Yosef 263:44).
At one time, men customarily accepted Shabbat during prayer with the recitation of “Bo’i kalla, Shabbat ha-malka” (“Enter O bride, O Shabbat queen”) in the liturgical poem Lekha Dodi. However, nowadays in many shuls they do not finish Lekha Dodi before shki’a. Accordingly, in order to fulfill the mitzva of tosefet Shabbat, the gabbai should announce after Minĥa: “Bo’i kalla, Shabbat ha-malka,” at which point everyone should accept Shabbat. If the gabbai does not make this announcement, then each individual should say to himself “Bo’i kalla, Shabbat ha-malka” or “Hineni mekabel al atzmi kedushat Shabbat” (“I hereby accept upon myself the sanctity of Shabbat”). Additionally, if one is afraid that the ĥazan will not complete the repetition of the Minĥa Amida before shki’a, he may whisper the declaration to himself during the repetition, to ensure that he not lose the opportunity to fulfill the mitzva of tosefet Shabbat.