Here are some guidelines on how to turn a modern western style "cowboy" hat into a Confederate slouch hat.
The first step is to read the article titled "Standard-issue Confederate Slouch Hats" by Bob McDonald (I will make a copy of this excellent article available for you to read, just let me know).
In this article, Mr. McDonald talks about the issue of Slouch hats in the Confederate Army, and gives a background of how and why civilian pattern hats were pressed into service and how they were often modified for Confederate use.
Mr. McDonald also describes in detail, two surviving original Confederate Slouch hats. Between this article and looking at the color photographs of other surviving original hats in the book "Time-Life, Echoes of Glory, Arms and Equipment of the Confederacy", this should give you an idea of what you want your slouch hat to look like.
In Mr. McDonald’s article and also visible in the pictures in the "Echoes of Glory" book, we see some of the ways that civilian hats were "converted" into a Confederate Slouch hat. Some of the commonly noticed modifications are:
The addition of binding or ribbon around the outer edge of the brim.
A cotton or ribbon band around the base of the crown for a hat band.
Often there was a brass uniform button (usually a State seal button) on the side of the crown with a corresponding buttonhole or loop on the brim to allow the side of the brim to be buttoned or looped up. Most often this was on the left side, but could also be on the right side.
Another common practice was to attach a pair of black or brown tassels to the right side of the hat (just over the right ear), at the base of the crown.
Note: This practice of using the tassels instead of hat cords was from the days of the prewar militia. Many militia companies wore a small pair of tassels, rather then the colored hat cords.
Black was the most common "branch-of-service" color for militia units, and prior to the change of the 1861 Federal Regulations, the Federal Army wore their hat cords, with the tassels to the right side of the hat. In 1861, this changed and the tassels of the hat cords were worn to the front of the hat.
Keep in mind that by the time that these changes were adopted, most of the Southern States had already seceded from the Union, so it would be logical for them to continue with the earlier militia practice, then conform to the new Federal Regulations.
Also, some Confederate and Southern State uniform regulations called for the use of colored hat cords and hat brass, however, in photos the use of either hat cords or hat brass is relatively rare.
Buying a Hat
The next step is to find an appropriate hat to modify. In photos, the darker colored hats seem to be more common. A black or medium to dark brown, is preferred and what I would recommend, but a tan or gray is also correct.
You also want to look for a hat that is made of 100% wool felt if possible, and the heavier the better. The lighter weight felt tends to lose its shape in the rain.
DO NOT get a hat with metal grommets or vents on the side of the crown, as these are not correct, and there is no good way to cover them. Also try to avoid the hats that have a welt sewn around the outer edge of the brim that contains a wire stiffener. This will need to be cut off and generally the reason that the wire stiffener is there in the first place, is that the felt is not thick enough or stiff enough to keep the shape of the brim when it is removed.
Modifying the Hat
To start modifying the hat, the first thing I do is remove the leather or vinyl sweat band and any other inside lining that may be there.
Next I take a look at the existing hat band around the base of the crown. On some original Slouch hats the hat band and/or the binding around the outer edge of the brim was made of "grosgrain" pattern ribbon of silk or satin.
Some of the modern western style hats have bands or binding that is similar to this. If the existing band and binding look "period" and not like polyester, then you may want to leave them on. Normally however, you will want to remove the existing hat band.
I then look at the outer edge of the brim. If it is too wide, you will probably want to trim it down. Most Confederate Slouch hats did not have very wide brims.
The article by Mr. McDonald gives the exact measurement of the brims on the two original hats and one is 2.75 inches and the other is 3.2 inches.
If the outer edge is folded over and seamed or if there is a wire stiffener there, you will want to trim these parts away.
Next, using black cotton bias tape (readily available, prepackaged at any fabric store), I put the fold of the tape against the outer edge of the brim, so that the tape covers about 1/4 to 3/8 of the top and bottom of the brim and I hand stitch the bias tape to the edge of the brim, using a simple running stitch all the way through the top and the bottom of the tape.
After the binding has been sewn on, I use 1 inch wide black cotton bias tape or quilt binding (also available prepackaged from any fabric store) for the hat band. You can use either a running stitch or a whip stitch, to sew the hat band around the base of the crown.
Next I use black cotton "extra wide double fold bias tape". I unfold it so that it is about 2 inches wide and then whip stitch it inside of the hat to the base of the crown to make the sweat band.
Note: Black is the most common color for the hat band and binding, whether the hat is black or not. It was not uncommon though, to see hat bands and bindings in the same color as the hat, or a darker shade of the same color of the hat (like a tan or medium brown hat with a dark brown hat band and binding, etc.).
At this point you may want or need to "block" or shape the hat. Your goal here is to shape the hat so that it looks the way you like it and so that it does NOT look like a modern cowboy hat.
With wool felt hats this is generally easily accomplished by simply getting the hat completely wet inside and out, shaping it the way you want it, then allowing it to dry a day or two in that position.
You might want to use cold water when you wet the hat, as 100% wool felt can shrink in warm or hot water, and you may want to use a Styrofoam "wig head" to help the hat to keep its shape while it is drying.
Now if you want, you can stop right there, as you have already gone farther then most west coast reenactors to convert your hat to a more authentic Confederate slouch. However, if you wish to make the hat a little fancier, there are a few other things that you can do.
You can sew a cotton lining in under the sweat band. The linings were often out of a woven check patterned cotton and would some times have a draw string at the top to better fit the head.
Another option, is to get a pair of small black rayon or silk tassels (again, readily available at almost any fabric store) and stitch them to the right side of the hat, at the base of the crown, with the ends right at the edge or slightly under the hat band.
If you wish to have one side of your hat up, you may also want to put a Virginia State seal brass uniform coat button on the crown and either make a loop at the outer edge of the brim so you can "loop the brim up" or cut a corresponding slit or make a hand-bound buttonhole near the edge of the brim so the brim can be buttoned up.
Two final notes; First, I would highly discourage use of hat cords (Officers excepted), other than the small black tassels. There are very few photos that show enlisted men wearing them, other than very early war.
Second, I would also highly discourage the wearing of hat brass for essentially the same reason. There are more photos showing troops wearing hat brass, then there are of them wearing hat cords, but likewise, most of these are early war. If you chose to wear hat brass, then at least keep it to either the letter "F" or the number "4".