Hi mate sounds daft but you just goto get down there and monitor the tides and where the ducks and geese are feeding,
I Would advise on going down at high tide roughly about 3 houres before it due to go out as the tide will push the duck up in the the channels further if not on the fret edge of the river allowing you to get a better shot and if you not got a dog last thing you want is for a bird to fall in open water. as you be swimming for it lol.
But if you take 2 or 3 decoys with you and no dout after the first shot the ducks take off gte your decoys out just on the edge of the channel an if you have a call start quacking and they will fly back in might take 5 10 mins but they circle arround allowing you to get a few more shots off, but if your not comfertable taking a shot better to leave it and let the bird land as that one bird will attract others in.
just a few tips mate any other probs or question feel free to ask