It's not easy, but once you understand what you're aiming for, it's not prohibitive in complexity. Carving a rifle stock from a blank is a lot like fitting a barreled action into a semi-inlet and the finishing is pretty much the same. The difference is that the semi-inlet stocks remove many of your options. Many choices you could make if you were starting from a blank were made by the pattern stock used by the semi-inlet maker. You typically lose most of your choices about cast, toe out, drop, etc. You won't have the option of a palm swell on the grip. You don't have much say about how long the forearm is going to be, and if the semi-inlet has a forearm tip, you might not like the length of that tip vs. the overall forearm length. Most semi-inlets won't accommodate custom bottom metal (eg, drop-box magazines).
There's rather a lot of things that semi-inlet stocks won't let you do, typically.
How to start?
Well, first you'd better start looking at a lot of custom rifle stocks. Take note of the geometry of the stock. Look at where lines start, where they go... extend lines and planes on the stock to see how dimensions and angles are arrived at. If you don't look at good custom rifle stocks, you'll be lacking a notion of where you're going. Starting in a vacuum, you could carve any one of a number of different types of stocks.
Find a stock blank that's fairly cheap to start with - like $100 to $200. Black walnut is often cheap[er], but it tends to be softer wood - you'll need to be careful when you're working it. Thin shelled (English) walnut holds details really nicely. Don't bother with more expensive blanks. You're going to make mistakes. Make them cheaply.
You'll need a good set of rasps. A hoof trimming rasp (get them down to the feed or tack store) is good for removing a lot of wood, but isn't absolutely necessary. The Nicholson 49 and 50 cabinetmaker's rasps
are essential, then a barrette file or two, a needle file, a chainsaw file, a pillar file... a straightedge at least 12" long, some jump gauges (which you'll most likely make yourself out of sheet metal), a good belt sander (for flattening the butt of the stock). A good, solid bandsaw can remove a lot of material close to your final side and top profiles if you're skilled in using a bandsaw.
Using a mill for the roughing out of the barrel channel and action area helps quite a bit. If you're willing to make fixtures for holding a stock blank (and you true one side and the top line of the stock in the forearm area), you can get rather a lot done by using a mill. You can rough out the action/barrel channel, then the magazine well. Some stocks will have little "gotcha" issues - eg, the rear tang screw on a 1903/A3 action isn't straight up & down. A Mauser 98 action doesn't have a lot of recoil bearing surfaces, so you kinda need those "lung" areas at the rear of the action.
You'll need a good selection of small chisels, gouges and scrapers to do a full wood/metal inlet job. You need, oh, at least three gouges - the smallest of which you can make from 3/32 drill rod and which is good for getting into the areas where you don't want to round out a corner. A 1/4 gouge and then a 1/2" gouge do a lot of the big removal. A 1/8" wide chisel gets used quite a bit. A 1/2" wide chisel is useful, especially for cutting "run stops" to prevent grain from picking up a cut and running it away from where you wanted it to go.
A "foot" chisel is used for working out the area where the recoil lug goes. You can make the narrow chisels out of screwdrivers. Heck, you can make the foot chisel out of a narrow screwdriver.
The gouges you can forge yourself from O-1 drill rod. Sharpen them with the bevel on the top side, not the bottom side of the gouge.
The easiest "nice rifle" action to inlet into a blank (without cheating by hogging out a lot of material and then slopping in a bunch of bedding compound) is the Model 70. The 1903/A3 and Mauser have much more in the way of features you have to worry about that burn up a lot of time the first time you do them.
Books:
Professional Stockmaking, by David L. Wesbrook gives some good overview and lots of photos.
Look for any books you can find by Alvin Linden. They're classics - but often
quite expensive in the out-of-print book market.
Edit:
Brownells sells wall-hanging prints by Jerry Fisher for Mauser and Model 70 stocks. Those prints give you some dimensions and ideas of proportions to use in making a stock.
And to add to Rod's point about "it's not numbers, it's hand and eye" -- to do a stock well, you need to develop a 'feel' for contours and profiles. When you're rounding the forearm, you want to not have "slab" sides or "corners" on it. The only way you really know you're done is to pick up the rifle as if you were going to mount it, then twist it back and forth in your left hand (assuming you're a right-handed shooter) as you glide your left hand up/down the forearm. It's a visceral sense that the stock has the correct shape up front, and then again, the sense of how you mount it and how it fits you (or the client) is what has to guide you in the buttstock. One tool that helps get shapes identical from side to side is a contour gauge, sorta like this:
http://www.generaltools.com/837--6-CONTOUR-GAGE_p_246.html
This gets you close to getting the side-to-side contours the same. You get all the pins straight, you push it down over the bottom of the stock (while the rifle is held upside-down in a vise) and then you "sink" the gauge most of the way down. Don't bottom out the pins. Carefully pick up the gauge, reverse it and very gently start lowering it onto the stock with the mark you're using as the centerline consistent with where you had it first. You'll see where you need to match up the contours.
When you look at Rod's tool collection there, yes, that's a lot of tools, but I'll bet some of Rod's tools there have ONE purpose. He probably made or modified some of those chisels/gouges/scrapers/etc when he got into a situation where he was cussing the tools he had, so he stopped and created or modified a tool to solve that ONE problem on the spot. That's how stock making goes. There's times you get into a corner (literally) when you're cussing a blue streak and you can see that nothing you have at hand is going to do anything but make a mess. You know if you make a mess, you likely won't be able to make it look right to get back out of it.
So you stop stock making and commence to making a tool to solve that one situation. You put it into the rack after you're done and then it's onto the next thing... There is NO woodworking supplier or shop who sells all the tools you need for high-level stock making. Most all woodworking tools are sharpened with the bevel on the bottom, not on the top of the tool, so they're "backwards" from what I like, and I'll knock off the edge and then re-grind the bevel on the top side of a gouge, etc.
Brownells or Midway won't have all the tools you need, either. Just get comfortable with making tools from O-1, heat treating them properly and sharpening them, because if you're going to do a nice job, the odds are nearly 100% certain that you're going to need a cutting tool you don't have and can't buy.