I would just use a 1ml syringe without a needle, and squirter it down the side of the tongue. Then give her access to water to drink. I would not dilute it.
If you administered on ypur own, wrap the duck on a towel and hol against your body with you non-dominant arm. Use the hand of that arm to told the bill open. Use your dominant hand to administer the doxy, being sure to press against the outside of the duck's neck so the dose goes down the side of the tongue and vocal chords. You need to avoid getting the dose between the vocal chords and into the lungs
