1
Look at the duck in the pond.
Point toward an imaginary pond to establish the location, then sign 'duck' by placing your hand near your mouth and opening and closing your fingers and thumb like a quacking bill.
2
I saw a yellow duck.
Sign 'I', 'see' (in the past tense), 'yellow', and then sign 'duck' by mimicking the large bill at your mouth with your fingers and thumb.
3
Please don't feed the ducks.
Sign 'please', 'don't', 'feed', and then sign 'duck'. Even though it's plural in English, you can just sign 'duck' once, or sweep the sign slightly to the side to indicate multiple ducks.