Harvesting requires a commander in the hex that the slaughter takes place and involves hitting an eligible unit with a single, non-combative strike which instantly croaks it. When the unit's corpse depops on the next turn, free rations pop for the units in the hex that the harvest took place. Situation Room Notes Page 23, cont'd
The rations produced by harvesting can be turned into provisions by the units in the hex after they pop. It's possible that the rations will still pop in the hex, even if friendly units moved on after the slaughter and did not end turn in that hex. It may also be possible for enemy units to take the hex that the slaughter was performed in before the next turn and obtain the rations when they pop.
It's possible that a procedure can also be performed to turn the corpse of a unit which died in combat into rations. This is supported by Jillian eating a horned sheep croaked either by doombats or Transylvitian warlords in First Intermission 15.
If a side exists which is known for cannibalism, it might be possible for them to harvest units which can talk for rations. This could also be the case if a natural ally exists which is known for cannibalism.