My favorite part is the shaping. After all the preliminary work on the metal and getting it inlet, with all that incurs, I finally get to tease that form out of the wood.
I have no set method for doing it, haven't really completed that many guns, but I have quite a bit of woodworking experience from the past. With a gun my general method is to begin at the butt and move forward, with a little skipping around here and there. You've got the butt plate, lock and triggers to guide you in the shaping, in fact those elements were put where they are because you were thinking of the final form of the gun, its shape, during the planning of the piece.
Not having a plan on paper, or at least detailed sketches, and therefore not having a clear idea where the various working components will go in relation to that ideal shape you have in your head, is a mistake. Unless, of course, you have years of experience building, anyone who has been building for a while in a specific style would probably dive right in on a stock blank with a clear idea of where they are going. If you are a beginner though, doing your research and planning is a must, and like Mike B. mentioned, seeing and handling the old guns is sure to boost your understanding of their form.
Planning is good, and rewarding, cutting into that blank can be daunting if you're not pretty sure of what you are after. Cut you must, but before you do have that shape firmly in mind, understand too how one area of the gun relates to the other, and how form follows function. Butt, wrist, lock area, forearm, muzzle area, all must flow together to make the whole, think of it as sculpture. The next opportunity you have for handling the old guns, or examining a nice contemporary, keep that in mind as you look the old girl over. When you know you will be getting the opportunity to view guns make a list of questions for yourself beforehand - then you will be doing "directed" looking. It is very easy to look at something and not actually see it, one reason for that is being new to the game, you simply don't know what to look for or even know what questions to ask. "How did this maker connect all the various elements of this gun to make a whole?" "What tools would I use to do that, how would I employ the tool?" Going to look at guns, be prepared.
In shaping there is an inescapable logic to things because of all the working components that go into making a firearm, your job as the builder is to connect and surround those components with an eye pleasing form, one thing flowing into the next with an eye to beauty. Decoration is just stuff on top of that.
Long winded aren't I...
dave