A technique I learned from Barelyproper, and have utilized to the most wondrous effect, is to use 3mil clear plastic painters dropcloth as pattern piece material. You don't have to cut the original paper pattern, it is extremely durable and at 3mil is sufficiently stiff to be able to easily work with. Lastly, when you pin it to fabric, it's clear so you can see what the heck you are doing!
The pattern pieces were traced out, and I made the modifications to both the lapels and to the front of the coat pieces, so to make sure the lapels ended in the chisel point instead of going all the way up to the shoulder seam.
The first mockup was cut out of white cotton muslin. Here is the back seam sewn together.
in this mockup we left out the tail pockets, as they require several extra steps to install, and do not alter the fit of the pattern. we did however leave the corrosponding split in the seam open so that we could check their positioning upon the wearing during the fitting.
The backs went together, and we sewed on one sleeve so that we could make the appropriate marks during the fitting for further adjustment. Here is the first mockup fully assembled from the front.
note the super high collar. It would eventually come down nearly an inch in height. Here is the mockup from the back
We had the first fitting with the gentleman, and adjusted the shoulder seam shorter by about an 1/2", the collar by 3/4" and we needed to add about 1" to the sleeve length. The most surprising bit was that we needed to add almost 5" in length to the torso!!!