English regimental coat collars in this period mimic the lapels in construction. Here is the under collar attached to the coat with the seam pressed open. The lining and interfacing peek in at the bottom.
As with armholes and sleeves, the neck opening of the coat has to be the same length as the long under collar edge. Too long is much easier to fix than too short.
The blue over collar overlaps the under collar by an 1/8". Here is the back turned up, the point is common on late 18th-century collars. To reduce bulk, the left and right collar pieces are sewn together edge to edge. Don't try this with anything but Kochan broadcloth. Make sure to fold the collar down BEFORE attaching the over collar. Pin it in a couple places--important to match the over and under collar fold so it doesn't pull the under collar outward.
The over collar covers the top pressed open under collar seam at the neck edge. The lining edge gets turned under, and covers the bottom under collar seam allowance. It is secured with an underhand hem stitch. Now comes the tricky part--the collar buttons to the top-most lapel button.
Mark and cut a horizontal buttonhole in each collar edge. They get the same treatment and tapes as the lapels. Next time faux pocket flaps.