I just give mine a tub of clean water and they clean themselves.
However, if you want to bathe, go down to the feed store and buy a tub of Orvis.  Mix one Tablespoon of Orvis into a gallon of warm water.  Stroke that into the feathers, in the direction that the feathers lie, so you don't break them.
Rinse really well.  At least 3 rinses and put a little bit of vinegar into the next to the last rinse to help cut any soap that remains. Then rinse again to get the vinegar off.  You want to be really certain that not even a single molecule of soap remains on the duck.
Don't get any soap into the duck's eyes.