Depending on how used to human contact your ducks are, the easier or harder it will be to catch them. If you have a good relationship with your duck then a bit of food will probably do the trick. If not, try to avoid chasing your duck around by walking it into a corner before trying to pick it up. This way it won't cause the duck and you much stress.
Ducks have more fragile feet and legs than chickens so the best way to get hold of a duck is either at the base of the neck or from above with a hand over each wing.
Once you have caught your duck from above, get it into a position so that you can carry it whilst supporting its body. Use one hand to support it from underneath by putting your index finger between its legs and securing the legs with your thumb and forefinger. Not only will this make your duck comfy it will also go someway to stopping their peddling legs and feet from scratching your hands.
With one hand under the duck, it is relatively easy to keep it quiet and do any healthchecks you need to. Handling your duck and getting it used to human contact will pay dividends in the long-run. Having a willing duck that's easy to catch and put to bed when you are in a hurry will make life much easier!