It is important to get the same "sight picture" every time you put the "rifle" to your face to aim. With M4/M16 rifles (5.56mm bore, U.S. military standard issue), you are instructed to put your nose on the charging handle so that your eye is exactly the same distance and position in relation to the rear sight aperture (your viewfinder) every single time. There are several other components that fit together in order to have an effective cheek-to-stock weld, aptly named because the intent is to have your cheek and the stock of the weapon "welded" together. They all come together like this:
Firing and Nonfiring Hand Grips: remember the hand positions discussed in the first chapter?
Rifle Butt Position: as mentioned before, there is no "rifle butt" of your camera, however combined with the Stock Weld, it is the same concept that results in your camera's positioning on your eye.
Elbow Location: I am actually really glad I was able to locate the above example series because of the final picture - notice everything we talked about in Body Position comes together here as well? His planted foot is flat on the ground, albeit with a sharper angle than the 90 degrees that I had mentioned before. 90 degrees is a good place to start with - once again, experiment with both! His non-firing (supporting) elbow is not "bone on bone" with his knee, and his firing (shooting) elbow is tucked into his body, not parallel to the ground, finishing with a forward lean "into his weapon."
But enough tangents - here's how to apply Sight Picture to your photography. On your camera, find a place that your nose can go to every time. For me, my right nostril is squeezed shut on the left edge of my camera combined with the top of the OVF cup bisecting my right eyebrow and pressing firmly against my eye socket. Every. Time. (This would be your cheek-to-stock weld). Then focus as necessary using your focus points and/or framing of the actual viewfinder.