How to Make Cornices

 Measuring the Window

What's Under the Cornice?

Getting the Lumber

What type of Lumber to Use: For the top headrail and the returns, we use #2 pine i.e. 1x4's, 1x6's, 1x8's etc. because it is one of the lesser expensive dry boards. (Buy dry lumber that is stored in a building. Using wet lumber or lumber that hasn't been dried can cause mold in your cornice.) If You don't mind paying a little more, you can also use clear grain fir, or any other lumber that is relatively straight. I only go to lumberyards where they let me choose my own lumber. While the At the lumber yard I hand select each piece to find boards that are strong and straight. 

Selecting the right boards. Sight down the length of each board from the end. You should be able to see any twists or curves in the board. Only use relatively straight lumber. (When choosing lumber, I try to find as straight as possible, but sometimes have to settle for what is available.) Next, watch out for knots. Small knots are OK, unless they are oozing pitch. Try not to use boards that have knots over 1/2 the width of the board.

The large cornice face boards: The front of the cornices are made from sheet boards. Many types are available: plywood, particle board, OSB strand board (also known as wafer board), etc. We use the OSB because it is usually a good price, but more importantly is usually pretty straight and flat. I would have preferred to use plywood, but I find the plywood it tends to twist and buckle more than OSB. Particle board is much heavier more fragile than OSB. When you bump the corners or staple into the particle board, corners and edges chip or break off much easier. The Particle board also makes for a much heavier cornice, which will make it more troublesome for your installer.

When choosing the sheet wood, get the sheets from stacks that are level. Sometimes if the sheetwood had tight straps, or if the sheets have been up on the pallets, the boards may be unnecessarily curved, which will make the cornices curved. It is just as important to sight down the sheets as with the other lumber. Stand up each piece and look for relatively flat and straight boards. (As I say this, just realize that very few are actually 100% flat and straight, but do the best you can.) If the sheets are too bent, if available, you might want to try several lumber yards to see any of the them store the lumber with more support under them (so the sheets will be flatter.)

Patterns

Building the frame

Padding

Underlining?

The Fabric

Patterns

Covering the Front

Trims & Cording

Lining The back

I mainly build the pine frame first, then the face boards on. I start by putting one side on first, beginning with the two top corner nails, and one nail at the bottom of the return. These starter nails hold the frame in place while I put the other side together. Then I put the face board on the other side snug up against the first one and  then nail it in place. After that I put the rest of the nails in. The middle face board joint will just naturally already be snug together, so I just staple the joint together. With a little practice, it's pretty easy.

if I join a head rail, I join it at the different place that where I join the face board (as shown in the bottom drawing below). But, if possible, I just try to use a one piece head rail.

Joining Cornice Face Boards
  About joining the face board (plywood, OSB, etc.) Over the years, I have tried joining the cornice faceboards in various ways. I used to use mending plates as Agnes said. Since then, with time and experimenting, I have settled on just butting the pieces together and  running a row of staples up and down both sides. I staple it so that one leg of the staple goes into each piece being joined, like this

The above picture shows a close-up of point "A" in the drawing below. The staples I used in the picture are Empire #7's.

I put the staples close together, as shown up and down both the front and the back. I put more staples near the bottom than at the top (because the headrail holds the top in place.) The thing to keep in mind is that the headrail holds the facing in place, and no one is walking or jumping on these cornices. (at least I hope not! )
On bay window cornices, at the joints I have laid webbing down the joints (half stapled on each side of the joints), that leaves a flexible joint (which is helpful for the installer if he needs to adjust it a little.) 

Try it!
Now, before you scoff at this, actually test it out. Fasten two pieces of a faceboard onto a headrail and join them as I had described. Then test the joint; see how strong it feels. If you did a good job of stapling, it should be solid.  I have joined many like this and never had a problem. I've never use the pressed paper, so don't know how it would work with that, but I would certainly test it out before deciding whether or not to use this method.