Lots of trial and even more error. I guess my first piece of advice would be to take your time carving the plug (original). Once you think you like it, set it down and dont look at it for a few weeks. If you like it after that build the mold. Your cast decoys are only going to look as good as the original so dont rush it.
More than likely you will not save money in the short term casting your own decoys. You may in the long run because if they are made right they will last a lifetime.
I would highly recommend 8lb density foam for the bodies and 16lb for the heads, especially if weight is not an issue. You can get away with using lighter foam but I've found that they are not quite as durable. We beat our stuff up pretty good. Id recommend overbuilding them for a few extra bucks rather than using the lighter foam. I'm not knocking anyone who uses 2,3, or 4 lb foam. I'm just giving advice from my personal experience. The heavier foam is a bit easier to work with as well.
If you have any specific questions feel free to ask. I'll tell you what I know.