There are relatively few books that talk about building cases, for several years I made reproductions of the colt casings and did quite a bit of research on them with the intent of one day writing a manual on the subject. French fitting was relatively rare with American guns and most of the original cased Colt sets I've seen with French fit interior were made for Colt by venders.
Colt, like any company made production changes to their cases over time. The earliest Colt casings were flat style interior, using slight indents and metal clips to keep pistol and accessories in place. They tended to have beveled lids, I've seen a Wells Fargo style baby dragoon in one of these cases, but for the most part you see Paterson pistols in them, though some of the Ehlers Pattersons were cased in the more common partion type casings common in the later Colt period.
The standard style Colt case of the 50's and 60's was a plain mahogany case with partitians. Fancier cases could were made of other woods such as Walnut, Rosewood and even oak, though oak was rare except in the English style. Fancier cases, and large ones often had secondary locks to keep the corners of the case down, and the really big ones had recessed pulls in the lids to carry.
English style cases also had a different interior, tended to be better constructed and used Oak most commonly, though other woods such as birds eye maple were occasionally used also. I tend to gravitate towards the English style since I find it more pleasing, but than you have the problem of locating a full size english flask to use with them. This style of case usually has a compartment in the corner with a cleaning rod included, something most of the American cases were without. Secondary catches on the English style seemed to be hooks most often, these are also easier to make than the catches found in the American style.
To get to your question about how to do it though, French fitting is basically creating a form that your pistol and accessories will fit into. You'll have to start with a piece of wood that is the inside dimensions of your case, lay out your gun and accessories on it where you want it, make some outlines, and than cut inside these lines. Remember that parts of the gun will go deeper than others, I would use a scroll saw and than add filler blocks afterwords to make the different depths. You'll find that most of the originals didn't go too crazy with this part, though you can add some contours with chisels and sandpaper if you want. You can line it in the traditional manner with baize, but you will have a hell of a time with that unless you do a lot of them, I found early on that the slikest way to do reproductions of any of these cases was with a flocking gun sold by places such as
www.woodcraft.com. You paint the surface with a special adhesive and than spray the fibers on with a flocking gun. You can mask of areas that you don't want coated, when done it is probably more durable than the original fabric used, and a heck of a lot neater in appearance. I strongly suggest this method, you can do things with it to make it difficult to see the difference between original methods.
As far as altering cases, make your own, the ones most commonly available on the market are a poor excuse for the originals. You can do much better yourself. You can do most of the work with a benchtop tablesaw, a router, hand drill and assorted hand tools, a planer would be nice, but the router can do the work of a planer and jointer with some separate jigs you can make at home.
