Read up on it. Google it. There is a lot of information out there on this topic (even if not on this site). For a new hunter the best way I often suggest is to use the alphabet. Set your spread in the shape of a U, or an X, or a J etc. The basic priniciple of any good spread is to have a kill zone/safe zone. You want the birds to feel "safe". With the U and the J setup you can guide the birds more or less to exactly where you want them to land. Generally speaking ducks will always takeoff and land INTO THE WIND. So facing your setup to maximize that is beneficial.
This topic can start getting real tricky and technical when your hunting in close proximity to others and/or you have seriously scouted your spot and know what the birds like. Examples would include... hunting on public land with 4 other groups within 350 yards of you. You have to do something to set your spread apart. That could mean more decoys, could mean less decoys, could mean more calling, could mean no calling. There is also a spot we hunt that we know the birds land in two spots on no matter what. We will strategically place our decoys so as to block off the areas we can't see from the blind and guide them to the two spots they seem to prefer.
There are lots of tips, tactics, and techniques in regards to placing decoys. But like I said earlier you want a Safe Zone and wind direction is very important. As far as the state of Washington goes.... I have had success with just 2 dozen decoys and I have had lights out shoots over 10 dozen decoys. Trial and error is the best learning tool I have ever found.
