The first stage of the marriage ceremony is the Erusin (betrothal), which begins with the Rabbi's recitation of the Beracha "Borei Peri Ha'gefen" followed by the Birkat Erusin (the Beracha over the Erusin). Strictly speaking, the Chatan (groom) should recite this Beracha, which constitutes the Birkat Ha'mitzva (Beracha recited before performing a Mitzva) required before performing the Mitzva of Erusin. However, so as not to embarrass grooms who may be unable to recite the Beracha, the custom evolved for the Rabbi to recite it in the Chatan's stead. But since the obligation to recite this Beracha in truth rests upon the Chatan, and the Rabbi merely recites it on his behalf, the Rabbi must instruct the Chatan before the Beracha to listen carefully and have in mind to fulfill his obligation through the Rabbi's recitation. Likewise, the Rabbi must have in mind during the recitation that the Beracha should fulfill the groom's obligation.
Needless to say, since the Chatan fulfills his obligation by listening to the Rabbi's recitation, he must not answer "Baruch Hu U'varuch Shemo" during the Beracha.
When the Rabbi completes the recitation of Birkat Erusin, he should taste some of the wine before giving it to the bride and groom to drink. The reason is that the Erusin ritual does not, strictly speaking, require a cup of wine, and therefore the Beracha of "Borei Peri Ha'gefen" recited over the Erusin is exclusively a Birkat Ha'nehenin – a Beracha recited over the drinking of the wine. The "Borei Peri Ha'gefen" recited during Erusin differs in this respect from the "Borei Peri Ha'gefen" recited during Kiddush, for example, which is part of the Birkat Ha'mitzva, given that the Mitzva of Kiddush requires drinking wine. Since the "Borei Peri Ha'gefen" recited under the Chupa constitutes only a Birkat Ha'nehenin, which one recites only if he eats or drinks the given food item, the Rabbi must drink a sip of wine after reciting the Berachot over the Erusin.
A second cup of wine is used under the Chupa, as well, over which the Beracha of "Borei Peri Ha'gefen" is recited as the first of the Sheva Berachot (seven special blessings in honor of the bride and groom). The accepted view among the authorities is that this recitation of "Borei Peri Ha'gefen" in fact constitutes a Birkat Ha'mitzva, and therefore the Rabbi who recites this Beracha over the second cup of wine need not take a sip of the wine. Nevertheless, Chacham Ovadia Yosef writes that it is advisable to take a sip from this cup, as well, to satisfy the opinion among the authorities viewing even this recitation of "Borei Peri Ha'gefen" as only a Birkat Ha'nehenin.
The Beracha of Erusin concludes with the phrase, "Mekadesh Et Amo Yisrael Al Yedei Chupa Ve'kiddushin." Our practice is to pronounce the final word with the letter Bet, rather than Vav, and it should thus be pronounced "Be'kiddushin." (This letter Bet is "Refuya," meaning, it does not contain a dot; our practice is to pronounce this letter as a softer "Bet" sound.)
The widespread practice is for the groom to break a glass after the recitation of the Sheva Berachot, in commemoration of the destruction of Jerusalem.
No Beracha may be recited in the presence of immodest dress, and therefore families conducting weddings must ensure that everybody standing under the Chupa is dressed properly, so that the Rabbi can recite the required blessings. It should also be noted that one of the Sheva Berachot declares, "She'ha'simcha Bi'me'ono" ("that joy is in His abode"), which indicates that the Shechina (divine presence) is present at a Jewish wedding. In order to for the Shechina to descend, however, the people in attendance must maintain an acceptable standard of Kedusha (sanctity), and immodest dress at a wedding thus prevents the Shechina from joining the celebration. In fact, some Rabbis maintain that if there is a lack of modesty at a wedding, the Beracha of "She'ha'simcha Bi'me'ono" must be omitted. Although we do not follow this opinion, it nevertheless behooves everyone to dress modestly always, but in particular when attending a wedding.
Summary: When the Rabbi recites the Beracha of Erusin under the Chupa, he and the groom must have in mind that the groom fulfills his obligation with regard to this Beracha through the Rabbi's recitation. The Rabbi should take a sip of the wine before handing the cup to the bride and groom. The Rabbi who recites the Beracha of "Borei Peri Ha'gefen" over the second cup of wine used under the Chupa does not, strictly speaking, have to take a sip of the wine, though it is advisable to do so. The proper pronunciation of the final phrase of the Birkat Erusin is "Chupa Be'kiddushin." The groom breaks a glass after the recitation of the Sheva Beracha, in commemoration of Jerusalem. It is critically important for everyone at the wedding, particularly those standing under the Chupa, to be dressed in accordance with the required standards of modesty.
(Taken from Halichot Olam, vol. 2, beginning from page 203)
