When I was giving oral meds to my ducks by syringe, I used a small barrel 1cc/1ml syringe and just gave a couple of drops at a time into the corner of the bill where the bill meets the feathers. It took my ducks about three days to start eating and drinking again and my most seriously injured duck took about a week to really resume eating normally. She lost a full pound, which was about 23% of her baseline weight, but she seems to be doing quite well, now.
Those last two days before she ate, I did give Nipper some liquified food by syringe, but again, just a couple of drops at a time, waiting for her to swallow in between. I was very aware of not wanting her to asperate it.