I gathered the sleeves at the hand and at the shoulder as well as the front part of the neckline.
I did it this way: I set the length of stitch to four milimeters, sewed along the edge, made one stitch up and sewed along the first line of stitches. Then I pulled the lower threads.
I had to add gores to the armpit. I serged the gores and the sleeves first. I sewed a gore to the sleeve, brought the needle down at the corner...
...turned the fabric and sewed plumb to the edge of the sleeve. I also turned the tip of the gore to me before sewing.
This allowed me to iron the gore like this so that I could sew the sleeves together without having any unwanted part of the triangle dangling out.
I pinned the gathered sleeves right side to the right side to the body.
I watched the lower edges of the sleeves to be at the same distance from the lower edge of the shirt.
I sewed the sleeves in. I pulled out the upper line of the gathering and serged the sleeve and the side of the body together.
I pinned the sleeves together. I payed attention especially at the starter point and the gore seam. I stopped sewing about ten centimeters from the end.
This part was reserved for the cuff slit. I ironed its edges flat.
I sewed the hips and pressed the seam allowances flat. I made slits here as well. I turned the lower edge right sides in and sewed a corner at the width of the seam allowance.
I turned the corners to the right side and I sewed the lower hem and the slits in one operation.