When it comes to seaming a sweater, I use the invisible seaming techniques outlined by Vicki Square in The Knitter’s Companion. I begin with the shoulder seams, then insert the sleeve caps into the armholes.
A set-in sleeve requires the pieces to be eased for a proper fit. I start by pinning the center of the sleeve cat to the shoulder seam, then pin the matching decreases at the armhole edges, then ease the rest in between. For pins I use old-fashioned roller pins designed to keep brush rollers (think hair) in place. You can still get these pins from Vermont Country Store. The pins are long, flexible, and blunt, all of which are idea when it comes to pinning knitwear.
I start the sleeve cap seam at the shoulder seam, working down the two sides separately.
I use the cast-on tails to sew the side seams from the lower edge to the armholes and to sew the sleeve seams from the cuffs to the armholes.
And then, I steam the seams and it’s all done!