The first thing I did was set up the sewing machine the teacher brought out for me to use. It was...old. I now really, really appreciate the features my own sewing machine has, especially including:
Step one was to fold each sleeve wrong sides together and sew a short line along the side where the two opposite ends of the rectangle are brought together (the red dotted line on the diagram labeled 1). First of all, I always pin the material so the heads of the pins will be facing in the direction of my right hand, so I can pull them out easily as I go. This is apparently completely opposite to the way the sewing teacher does it...I guess she pulls them out with her left hand? I'm not sure. In any case, she mentioned how weird it was for her to put the pins in backward. Next I started up the sewing machine...which made a horrendous grinding noise. I was afraid at first that I had broken it, but I guess it was supposed to sound that way...? The teacher didn't seem to think anything of it.
Once the first line was sewn, we flipped the sleeves inside out. Unlike most other clothes, in which the fabric is folded directly to the seam line, traditional Japanese clothes apparently have a bit of buffer zone around the seam called a kise. Thus, instead of the seam looking like the top of the letter A, it looked more like the letter M, with the actual seam line in the valley.
I then proceeded to sew the curve of the sleeve, the part that dangles out farthest from the body in the finished product (the red dotted line in the diagram labeled 2). As soon as I did that, I was told that the machine-sewing portion was finished. The next step was to sew, by hand, two arcs, each a little over 1/8" apart working out from the curve. (In the diagram they look like straight lines, but I was instructed to sew parallel to the curve itself.) This took me quite a long time, because I had to keep stopping to wipe my hands dry. Sewing by hand is definitely not my strong point. By the time I finished one sleeve (the teacher did the other), the hour was up and it was time to call it a day.