You need to make poison at one of the two alchemy huts. Then 'pour in' this poison into the cooking pots that the NPCs collect their daily routine foods from.
You can steal the apples and eat them if you need to top up your health and/or satiation level.
There are at least 3 cooking pots in the camps - two in the Cuman part and one (or more) in the local bandit area. There are also multiple arrow barrels. I'm not sure what effect poisoning different proportions (or burning fewer arrows) has on the preparation results. Generally I've been underwhelmed by how much influence significant efforts to cull numbers, perform all sabotage and thoroughly scout all parts of the camp have.
Burning the hay during the battle can reduce a small amount of archery from the church, but is also underwhelming.