I turned the lining of the armhole right side out. I pressed the edge for the seam to appear inside the garment.
I attached the seam allowances by hand. I sewed with herringbone stitch and caught only one thread of each layer. This way it shouldn't be visible at the right side.
Or a machine alternative. I used it for long straight parts - the lower hem and the center-front lining.
It's called blind stitch. You fold the hem under the garment. You sew at the seam allowance, at the serging and every fifth stitch goes to the left and catches one thread of the fabric.