how to attach the sleeves to the back when knitting Arwen in one piece

Well, one way, anyway. It's a little difficult to explain, but um. I joined new yarn at the right armpit (with a nice long tail, because the armpits needed some tightening anyway) and knitted across to the last stitch. I slipped that last stitch, then picked up one stitch in the selvedge edge of the back of the left sleeve and passed the slipped stitch over the picked-up stitch. I always slip my selvedge stitches, so I had a nice clean edge to work with.

As you can see, there is a visible line, but it's not nearly as lumpy as a sewn seam would be. Erm, a seam sewn by me, I mean. I'm sure y'all are much better at seam-sewing than I am.