The main button strip down the middle of the shirt and the ones on either side on the cuffs are, essentially, low relief carvings: line in, lower the surounding ground, set in and finish the ground to the junctions. You'll see this approach elsewhere on Woodcarving Workshops, in the 3 Leaves project, for example.
Be accurate: the illusion of the shirt depends on a sense of precision in the tailoring!
Scraper: If you haven't already seen the video, I show you how to make and use a woodcarver's scraper here.