I'm having a hard time developing a decent way. With chickens, I put them in the cone and slit their throats. These turkeys I'm working on now fit in the cone, but it takes them at least half an hour to bleed out and die, and I don't feel that that's very humane.
So this last one, after he still wasn't dead, I went and got me a stump and a good hatchet. Turns out that's a little tricky to do by oneself; if they move at all, and they do, good luck getting that first stroke in the right spot. And then he bruised his wings flapping around, which they don't do in the cone. I think I'm just going to have to accept that this is a two-person job.
Sure do wish I had a ten-gallon stock pot for scalding too. They do not fit in the five gallon.