Sleeves can be tricky if they aren't lined up right. To make things easier I've pinned the body of the sweater down, measured and marked where the end of the sleeves will be.
Another important mark to make is the middle of the sleeve. This will line up with the shoulder seam. Since the sleeves have a bound off edge I'll use the ladder stitch.
I always leave a long thread when finishing an piece so I can use it to sew the pieces together later. Here, even though the thread is already on this side I want to come up through the piece as though I were starting the ladder stitch.
Then come over to the other side. Find the stitches that go up in a row and sew these. That way your seam won't go back and forth but stay neat.