PO means that it’s to be administered orally, like swallowing a pill.
SC means subcutaneously, injected under the skin.
and IM means intramuscularly, injected into the muscle.
The last two don’t really matter because you would need liquid antibiotics which I don’t even know where to get.
q12 - 24 means to administer the dosage every 12 hours to 24 hours. A vet would be best able to determine if the duck should get a lower dosage rate “24 hrs” or a higher one “12hrs” usually that depends on what bacteria is cultured and how aggressive/resistant to treatments it is, which without testing is impossible to say.
50 mg/L drinking water means how many milligrams to 1 liter you would administer if you were adding the medicine to drinking every day.
So the dosage is 10 to 15 mg per kg, so if you’re in the U.S like me where everything’s in pounds you first need to convert the weight of your duck to kilograms, then add the mg dosage to your result and divide the pill to that amount.
I don’t know what breed you have or what they weigh but a typical pekin is about 8 pounds so converted to kilograms that’s 3.6 kg so the dosage for an 8 pound duck would be 36 mg every 12 to 24 hours is you’re going with the lower range 10mg per kg for example.
What I don’t know is how many days your duck should be taking these, two weeks tends to be what my birds have been prescribed antibiotics but those were for very different conditions, cipro tends to start having an effect, starts working at 5 to 7 days, sometimes it’s hardly noticable, but if you think your duck isn’t improving at all or getting worse it can mean that the bacteria isn’t reacting to the medication and you need to try another one or try to get a vet.