To add a Duck or a Goose Pond on your farm, the first thing to do is purchase 4 tiles of water. To get water tiles, click on the big yellow arrow icon at the bottom of your screen -> click on the plot with the hand icon.

Click on the 4th icon from the left side, and you will be able to find a water tile.

Place the tiles of water 2x2 to form a larger square. Once they have been placed, go to the Store:

and purchase the Green Duck Pond (or the Goose Pond).

Place the pond on top of the 4 tiles of water. When placed correct, the pond will turn green and you can press the green tick to successfully place the Duck Pond on your farm.