Duck hunting is available in the marshes east of Thebes, just to the east of the south gates. You will need a bow or crossbow to hunt, so be sure to bring one with you! The next thing you will need are ducks, so head out into the marsh and find them.
As you walk through the marsh, you will see ducks in groups or sitting alone. Looking at the ducks will tell you what direction they are flying, how fast they are flying and how far away they are.
Once you find the ducks you wish to shoot, you may either wait and hope for them to fly closer or take aim and hope you're good enough. Aiming at the ducks for the first time will center your crossbow directly on on the group of ducks. Based on their speed, distance and direction, you will want to adjust your aim by typing aim up/down/left/right. Each time you adjust your aim it will move incremently, so you may need to do it several times to get it just right.
For example, if a group is flying very far away to the east very quickly, you will need to adjust your aim up quite a ways to make sure the arrow flies far enough and then to the right quite a ways to make sure it meets the target if it does fly far enough.
When you think you have your aim correct then fire. Either you will hit and bring down a duck or the arrow will miss, giving you hints on what you did wrong. For instance: "The arrow sails below the ducks and barely to the right." means that you did not aim up enough and aimed to the right too far, but just barely. Adjust your aim accordingly and try again.
Sometimes ducks will be just sitting in a pool of water, not even flying. These are easy prey and should be taken advantage of. The closer the duck, the better the shot, so choose your targets carefully and be patient if the group is flying toward you. If the group is flying farther away then you better act fast.
Watch the room messaging for information on the duck's movements and changes in direction. This plays a crucial role in your aiming.
Now once you manage to down a duck, you can't just run over and pick it up yourself. You need to stick to the tracks or you might get mired in the swamp and then you'll have more problems than just bad aim. This is where your trusty retriever comes in to play. Simply whistle and you will call your dog. Then tell <dog> to fetch and it will run out and bring back your kill. Now you will be able to pick it up. You can also tell your dog to stay, jump or leave.