Invisible seam

Invisible stitching allows you to join two items knitted in stockinette stitch without visually breaking the stitch.

To make it, thread a thread the same color as the knitting into a wool needle.

Work on the spot.
In the 2 pieces or 2 sides to be joined, take the horizontal thread between the 1st and 2nd stitch and pull the seam thread enough to bring the 2 edges together, in order to make the seam invisible.
If the edge is particularly uneven, take the horizontal thread between the 2nd and 3rd stitch.

The shoulders can also be sewn together with invisible stitching.
Then take the 2 vertical strands of a stitch one row from the edge of the work.