Most domestics don't fly which makes it unnecessary. Just FYI.
You can do it yourself, but it involves cutting off the first joint of the wing. I think they make special nippers for it. You might see if a vet in your area can do it for you with a sterile environment. THere will be aftercare involved as well to prevent infection.
Last year we pinioned our chicks. We used fingernail clippers, and did it when they were only two days old or so.
You clip of the last joint, so it is actually cutting off bone. It is the place that would be the 'thumb' on your hand. Let me see it I can find an example-
OK, you have the elbow joint, then the wrist joint, and on the wrist joint there is the hand bone and the thumb bone. the hand bone has the flight feathers on it, and the thumb joint is short and has (I think) the outermost three or so flight feathers. That is the one you clip.
Now they are grown and you cant tell by looking at them unless you spread the wings out. They cant fly, but we are not doing it again this year. I will just clip feathers if I need to. We had one almost bleed to death, we must have got too much bone. They all bled, and then they picked at each other. So it is up to you. If you do it get some stop bleed stuff.
See if you can find a diagram or something, that would make it easier to see what you are doing. You dont what to mess it up and kill them or maim them.
If you get them young and they grow up there, they don't usually go far anyway. Sometimes being able to fly is what will save them from a predator. Most places where you could keep ducks it wouldn't be that big a deal if they did fly a bit. It's not like they're mallards or some other variety that might actually migrate.
I had Khaki Campbells, they can fly a little, (not as well as a Muscovey) but it wasn't a problem. My chickens and guineas all fly, they stick around, and go in at night.
The idea of cutting off part of an appendage, with no anesthesia, just makes me cringe. It would seem like cutting off a hand. I could not ever do this.
If you have to keep them inside a pen and are afraid they're fly out, you could add a chicken wire cover to the pen, rather than cutting off the wing tips.