There are 5 ducks to collect.
Duck 1At the beginning of the game just past the fountain you will enter the manor, opposite the entrance door is a duck on the window ledge.
Duck 2When you enter the kitchen, go into the back room that leads to the outside garden and in that room is a duck on the unit opposite the door
Duck 3Again entering the kitchen this time take the left door and open the secret entrance, follow it through the next door and a duck will be on your right side slightly hidden behind the crate.
Duck 4This will be in the attic at the top on the far left side sitting on a crate, it should be opposite the seeds.
Duck 5When you enter the boat house you will come to a potting table, to the left of it on the crates is the final duck for this achievement.
If you need more help with this you can use my video guide below to find all ducks, they are all timestamped so you can skip right to them: