#1- Lay the sweater body tube flat, with the *seam* (the beginning of the round) to one side. Measure the sleeve width at the top of the sleeve, and then measure that length from the top of the sweater body tube and mark with a pin. Repeat on the other side.
#2- Sew along the cutting line with highly contrasting waste yarn, stopping about 2-3 sts from the pin.
#3- Zig-Zag stitch along either side of the waste thread (or reinforce the new steek in your preferred manner)
#4- Zig-Zag across bottom of the cutting line, 2-3 sts above the pin
#5- Cut armhole open between the zig-zag stitching, to within 1 stitch of the end zig-zag stitches
#6- Both armholes cut open
#7- If you know how wide you want your shoulder seams to be, sew them now. Otherwise, just sew a couple of inches in from the armhole opening. (You can figure out your shoulder width when you do the neck shaping).
#8- Fold the new steek edge in 3-4 stitches, and whip stitch in place.
#9- Pin the sleeve in place and sew the sleeve in the armhole
#10- Sleeve sewn in place. Ease the sleeve around the lower opening edge.