During the Civil War period, almost all buttonholes were sewn by hand¹ .  When buttonholes are sewn with the buttonhole stitch it prevents the buttonhole from fraying and reinforces the opening.

1. Align the buttonhole in the direction of stress placed on the button.  Buttonholes on the front of coats and dresses are vertical.  Sleeve and shoulder strap buttonholes are horizontal.  The buttonhole begins one button length from the edge of the garment.  The length of the buttonhole is slightly longer than the button’s diameter.  Mark the buttonhole on the fabric.
2. Cut buttonhole with embroidery or buttonhole scissors.
3. Using regular thread for fine fabrics and buttonhole twist for heavier fabrics, with needle and thread, overcast edge of buttonhole.
   
   "Overcastting is a slanting stitch, used to keep raw edges from raveling.  In taking the stitch the needle should always point toward the left shoulder.  Hold the materials loosely in the left hand.  Do not use a knot, but turn the end of the thread to the left and take the first tow stitches over to fasten.  Make the stitches one-eight of an inch apart and one-eighth of an inch deep" (Harris 4).
4. Place fabric over finger and beginning near one end, sew around buttonhole with buttonhole stitches, spacing stitches two threads apart.  Use care around ends.
   
   To make the buttonhole stitch, "The needle must go in on the wrong side, and be brought out on the right, five threads down.  To make the stitch, the needle is passed through the loop, before it is tightened or drawn close"
   
   "Observe that you keep your work evenly by the thread, and do not turn the corners too soon; the needle should be put in between every two threads, else the work will not be thick enough" (A Lady 3).
   
   
5. To finish, weave thread under the completed stitches on the wrong side of the garment.

To get the most strength, it is important to wax the thread before use. The waxing will strengthen the thread and prevent twisting while sewing the buttonhole.  I think the proper thread for medium to heavy weight fabrics, would have still been the "Silk buttonhole twist", size 8/3 D, E or F.

¹ Henry Alonzo House invented a commercial buttonhole machine in 1862, but its use was reserved primarily for military clothing production during the Civil War.  (Bridgeport History Links) 

Courtesy of Fannie and Vera's