Or, more precisely, clipping a chicken’s wing – singular, because this is what actually stops them escaping. Cut them both and you have a good chance they’ll still be able to fly. That’s the received wisdom anyway, I’ve never seen it myself.
The actual clipping of a chicken’s wing is fairly straightforward as this video shows, but if yours are as untame as mine, it’s the catching them that’s the issue. I usually do this at night, when they’re all tucked up in their house, but that doesn’t make for great filming.
You’ll need to do this when the chickens a few months old, once their flight feathers have grown, and then after every moult, when their previously-clipped wing feathers grow back.