Here's an example of a repair I've done.
This stock was on my friend Nic's P14 rifle that he had. It was broken quite badly right through the left side of the stock and up through the rear section where the action screw is.
The crack was about a 1/8" apart which made the stock fit terrible.
Anyhow here's a pic of the damage.
My first step before I do anything is to look over the damage and decide what my best course or action is going to be and also how/where I'm going to clamp the crack once I glue it.
Second step is to clean the oil from the wood with brake clean. I usually use a full can over a day or so and keep spraying it in til I feel that the stock is clean.
Next I let the stock dry over night.
Next step is to mix up epoxy which will be used to glue the stock back together. If you are concerned about the epoxy showing then mix in some fine sawdust to the mixture. The sawdust will take stain and therefore will help in hiding the epoxy.
Then I use a suitable applicator (based on the size of the crack, can be your finger, a Popsicle stick whatever works) to get the glue in the crack.
Once the epoxy is in the crack good I clamp it.
In the last pic you'll notice that the epoxy is smeared all over the stock around the crack. That's no big deal cause once the epoxy starts to cure (about 3-5 mins with the stuff I use) I simply roll it off with my fingers which takes care of it, the key is you have to do that BEFORE it gets too dry!!
Then it looks like this.
Next step in a really bad crack such as this one is to reinforce the damage. In this repair I used 2 screws which I hid under the action and bolt release so it would be invisible. I drill holes then put epoxy on the screws and gentally screw them in. I let them dry and then cut off the heads. Then I use a dremel to smooth the heads flush with the wood.
Here's the finished project after some light sanding with 600-1000 grit, staining and a good amount of linseed oil.
Hope that helped!!