"What are the issues?", you ask- well, first an explanation of the standard jacket collar and how the top fabric is traditionally applied.
The traditional suit undercollar is a one piece pattern in which one edge joins to the neckline of the coat and folds along a predetermined roll line, to form the fall of the collar. These have a stand that usually ranges from 2.5cm to 3.5cm deep. The fall of the collar is slightly deeper than the stand. The undercollar is cut on the bias so it can be folded and stretched a little to travel smoothly around the neckline of the jacket. The roll line of the collar is the transition between the stand and the fall, and it is usually stitched so that it will not stretch out, while both the edge of the stand and fall are stitched and pressed to allow some controlled amount of stretching to occur. If the outer edge of the collar is too tight the collar will tend to roll above the intended roll line and if it is too long it will do the opposite and want to lie flatter on the garment than intended.
The fabric that is applied to cover the collar is cut in one piece on the straight grain.
Traditionally, tailors would then stretch the top collar fabric along the outer edge, and shrink the fabric that lay along the roll line in order to make that straight grain fabric bend smoothly around the outer edge of the collar, sit along the roll line without rippling, and finish to the inside neckline in a smooth way.
You need wool (and preferably not that papery wool) in order to be most successful at doing this.
If you look at a modern suit jacket, you will see that the top collar fabric is now cut in two pieces with a separate stand and fall section to allow the collars to sit clean.
Now to the issues.................
In the case of the 19th century coats, the stand and fall are much deeper. The fall especially is quite a bit deeper than the stand and therefore the outer edge of the collar needs to splay a great deal to sit properly.
Historically you will often see the top collar fabric cut with a seam at the centre back, or on the bias like a shawl collar, or with both bias and seam.
We didn't want this on our coats, the wools are not easily manipulated and I've been given both suede for one top collar and velvet for another, so I needed to make a top collar pattern with a seamed stand.
The first step was to make the undercollar pattern in canvas and melton, cut on the bias.
The undercollar needed to stretch along the neck edge just a little to fit, the roll line was held from stretching by stitching it by machine, and the outer edge needed to be stretched a lot to sit well on the coat and not push the roll line higher than intended.
I marked a seam line inside the roll line, added two matching points and cut the pattern apart.
I then slashed from the outer edge just to the original roll line, and made small clips from the new seam to the roll line. Open out the slashes the amount required, taking care to spread the collar most in the area of the side of the neck, and allowing the pattern to overlap itself along the small clips you have made in the new seam line. This keeps the area at the roll line from growing. It does shorten the seam a tidge but continue on.
The stand of the collar also needs a little bit of slashing and spreading along the neck edge to fit the stretched shape of the undercollar.
Retrace your pattern and smooth out the new lines/shapes.
The seam line of the fall will be slightly shorter than the seam line of the stand and most of that discrepancy will be in the side neck area, which happens to now be on the bias (remember the top collar fabric is cut on the straight). When stitching the seam, you will stretch that area to fit the seam of the stand.
Press it open, keep that seam allowance quite small -1/4" maybe. Clip only if needed to get it to lay flat, and then you can apply your top collar without having to manipulate the fabric to get it to fit.
I hope you can see the details in my hand drawn diagram- it is a little faintly drawn, I will have to clean it up and draw it properly sometime..... but not tonight.