[Mismost]

I'm gonna take a different road here. Gun fit is comfort, first and foremost. If the gun isn't comfortable to shoot, then you just won't shoot it after a while! I won't be beaten to death by any firearm.
Now, you buy a good suit the first thing they do is "fit it" to you. Think of your shotgun as a great new suit and have it fitted. You will look better, shoot better, and be more comfortable. Remember this...the gun is the cheapest part of the shooting games. Spend your money wisely.
BTW, gun fitting is not just for shotguns...it works on rifles too! I've got a Kimber 22 Target rifle in the safe that I put a belt sander to the left side of the stock and dished out a spot for the side of my face to fit into....NOW, my eye lines up right with sights..no craning or cocking the head, no cramped neck muscles, and better scores to boot. Looks bad, works great and I'll take "works great" over "looks good" any day.

 

[Mismost]

Neil...Oh, you want an answer to the orginal question! OK, you fit a shotgun by adjusting the stock dimensions/posistions to fit your body and type of shooting you do.

You do this by adding or subtracting length of pull, comb height, pitch, cast on or off, among others. All have some bearing on the other! Sometimes it's as simple as a few strips of mole skin stuck on top of the comb to raise your eye above the rear of the gun, so you see better and stop lifting your head and so shoot better.

The point is to form the gun to your form and in so doing make shooting it much more comfortable and accurate. In my mind is an art form and a blessing!