The custom of Israel is to take aravot on the seventh day, to commemorate the Temple, and to beat them on the ground or something else, as the prophets instituted in Temple times (MT, Laws of Shofar, Sukka, and Lulav 7:22). The Amora’im disagree as to whether the prophets introduced this practice as an enactment, in which case a berakha is recited, or as a custom, in which case no berakha is recited (Sukka 42a-b with Rashi). The accepted practical ruling is that it is a custom, and no berakha is recited (SA 664:2).
The aravot need to be 3 tefaḥim long (like the aravot bundled with the lulav). Technically, one arava is enough, but it is customary nowadays to follow Arizal and take five aravot (SA 664:4; MB ad loc. 16). Some prefer not to tie the five aravot together. Others prefer to tie them, and this is the custom (MB 664:17; Kaf Ha-ḥayim ad loc. 35).
Although the aravot used for the mitzva of the four species are invalid if most of its leaves have fallen off, aravot used for the custom of beating are acceptable as long as one leaf remains on each arava. Nevertheless, one should not be lenient. Rather, one should beautify the custom and use nice aravot that are valid for use with the lulav (SA and Rema 664:4; Kaf Ha-ḥayim ad loc. 34).
Le-khatḥila, one should take new aravot for beating, rather than those that have already been used by someone else. However, technically, many people can fulfill the custom with the same aravot. The beating itself does not disqualify them. As long as each is 3 tefaḥim long and has at least one remaining leaf, they may be used multiple times to fulfill the custom.
The aravot are beaten two or three times on the ground or something else (Rambam; SA 664:4). According to Arizal, the custom is to beat the aravot five times on the ground. Some Aḥaronim write that it is preferable to first beat the aravot on the ground in accordance with Arizal’s custom and then beat them on pews or other furniture, because some maintain that it is better when the beating tears off leaves, which is more likely to happen when beating them on furniture (Bikurei Yaakov 664:16; MB ad loc. 19). Those who follow Arizal meticulously beat the aravot five times on unpaved ground and make sure that the aravot remain kosher enough to be used for a lulav throughout the beating (Kaf Ha-ḥayim 664:37).
In most communities, the four species are not taken together with the aravot for beating. Rather, the seven hakafot and the subsequent prayers and liturgical poems are conducted while holding the four species. Then the four species are put aside and the aravot picked up. Additional prayers and liturgical poems are then recited, and the service concludes with the beating of the aravot (Rema 664:7; MB ad loc. 26 based on Arizal; Kaf Ha-ḥayim ad loc. 32). The Yemenite custom is to hold the four species plus the aravot during the hakafot of Hoshana Rabba (as described in SA 664:3).
One cannot fulfill this obligation using aravot that are still bundled with the lulav. However, if the aravot are removed from the bundle after the four species have been taken, one may use them for the mitzva of beating aravot (SA 664:5; MB ad loc. 21).