You need to let one of your ducks go broody first. And the out the eggs where she has decided she wants to have babies....
If they are like mine, they can often decide that they want to have babies in the stupidest of places... i have one nesting in my ourdoor fireplace! -thank god I didnt light the fire!
The other thing you can try, that I did with one of my girls when I noticed she was going broody, was I then shut her into my largest house with run and nest box, along with a pool, her favourite foods, and her favourite drake...
She then of course had no option but to lay in the nest box, and after she had laid engh for her nest, she sat on them there, and than raised the ducklings in the same place.
Can I ask what breed your ducks are? Some are better mums than others? For example I have silver appleyards who are constantly going broody, but are rubbish mums, they either get off the nest right before the ducklings are due to hatch, or abandon them five minutes after they hatch, while my khaki campbells, who indont think are reputed to be great mums, have been far more reliable, but not perfect.
Anyway, with domestics, if its their first time, try and get them locked in a house with a run, (obviously put a favouried drake in so the eggs are fertile!) to nest, otherwise you may find yourself with eggs close to hatch day, and suddenly mum (who looked you in the eye and swore she wanted babies more than anything else in the entire world, even green peas) gets off the nest and leaves them to die....
Either that or make sure you have multiple backup ducks, or broody chickens, or an incubator.
While some of my domestic ducks are awesome mums, ive lost count of how many ducklings this year have ended up being hatched in my bator, or raised by me, thanks to mums losing interest etc... There is one beside me right now...being looked after by an older duck who is also one I had to hatch when her mum got off the nest a few days before she was due to hatch....
Once you have a duck who is broody, and showing nesting behaviour, you can basically put whatever eggs you want under her, i have several who indont wish to breed, who I have put eggs from ducks I do want to breed under, they are happy to sit on whatever... If you put a turtle eggs under them theyd happily hatch it lol.
Actually I have some ducks who sat on eggs for the 28 days and nothing hatched (most were not fertile unfortunately) and after the failed nest, they actually moved the failed eggs out, then took eggs laid by other ducks nearby, and moved them into their nest, and are now sitting on those... So not only will ducks sit on other ducks eggs, they will knowingly sit on other ducks eggs or anything that might hatch, when they are broody.
Most of mine seem to go broody from mid-late spring. Though the ones that sit later seem to be more likely to abandon the nest.