961. Shaking the Lulav
137:1 One takes the lulav, joined together with the hadassim and aravos, in his right hand with the spine of the lulav facing him; he takes the esrog in his left hand. Because we need to recite a bracha before performing a mitzvah, and since the esrog must be held the way it grows in order to perform the mitzvah - namely, with the stem where it was cut from the tree facing downwards and the pitom facing up - therefore, when taking the esrog, we take it upside-down before reciting the bracha, with the stem upwards and the pitom downwards. In this way, one does not yet fulfill the mitzvah. We recite the bracha of "al netilas lulav" while standing. (If one was sitting, he fulfills his obligation after the fact – Mishnah Brurah 651:27.) The lulav is named in the bracha because it is the tallest, and therefore the most prominent, of the four species.
On the first day of Succos, we also recite the bracha of shehechiyanu. If the first day was Shabbos, when we don't take the lulav, then shehechiyanu is recited on the second day. (If one neglected to say shehechiyanu on the first days, he may still do so later, even on the last day of Succos – MB 651:29.) After reciting the bracha, one turns the esrog right side-up and brings it close to the lulav so there won't be any gap between them. He waves the four species in all four directions as follows: east, south, west, north, up, and down. (This assumes that one is facing the aron, which is typically in the east of a shul, hence to the front, to the right, behind one, then his left, up and down.) (If he didn’t wave, he still fulfills his obligation but he should go back and wave – MB 651:49.) One does likewise when waving the four species during Hallel and during the procession for Hoshanos, being careful to bring the esrog near the lulav so that there won't be any gap between them. If one reversed things and took the esrog in his right and the lulav in his left, he goes back and takes them again but he does not repeat the bracha.
137:2 A left-handed person takes the lulav in his dominant hand and the esrog in his weaker hand; if he took them improperly, he must go back and take them again, without reciting a bracha. An ambidextrous person is the same as a right-handed person.