I suspect the reason you didn't get an answer the first time was because it's seemingly self-explanatory. You'd see men literally beat other men (most of whom probably were friends, it being a legion) to death with their fists - which is as far as the sources go in explaining the event to us. This is how Polybios (who likely never saw this happen) describes the procedure:

[---] The Tribune assembles the legion, calls the defaulters to the front, and, after administering a sharp rebuke, selects five or eight or twenty out of them by lot, so that those selected should be about a tenth of those who have been guilty of the act of cowardice. These selected are punished with the fustuarium (Eng.: a cudgelling to death) without mercy; the rest are put on rations of barley instead of wheat, and are ordered to take up their quarters outside the vallum and the protection of the camp. As all are equally in danger of having the lot fall on them, and as all alike who escape that, are made a conspicuous example of by having their rations of barley, the best possible means are thus taken to inspire fear for the future, and to correct the mischief which has actually occurred. (Plb.6.38)

Please don't ask people to make things up on here. :)