It sounds to me like OP wants to harvest the down for later use. Unless you have a way to secure the duck firmly, upside-down. this will be really hard to do. So figure that out and then try it. I have never dressed out a duck, but I hear they're very challenging to pluck--one reason I'm vacillating about trying them as meat birds.
I'm not sure I'd try to collect the down. Modern insulating garments/comforters are more effective and less expensive and easier to care for than down. I'm not sure down is still as valuable as it once was. I'm not sure there's any market for it except maybe as a niche craft material. Before going to all the trouble, you may want to research this. Of course if you want the down for some personal use you have in mind, that may make it worth your trouble.
From what I've read, if I wanted it, I would try collecting it from my geese when I notice them beginning to molt. I'm told this doesn't seem at all traumatic to them if the time is right. If I found this to appear painful, then I would stop.
When the down is wet, it will stick to your fingers--it's a mess with chickens and turkeys (the big feathers don't cause this kind of trouble), so I can only imagine it with waterfowl. If you plan to pluck rather than skin, you will want to scald. I partially dry plucked a turkey several weeks ago, because my daughter wanted quills for pens and assorted other feathers for crafts. I would not want to have to do that very often.
I would absolutely want to scald for waterfowl, and perhaps dip in paraffin for the removal of the finer feathers. From the YouTube videos I've watched, it doesn't look like anyone gets all the fine plumage out as one does with turks and chickens.