The next step to pretty set-in sleeves is seaming. Done properly, a sleeve seam can be a thing of beauty, and if you have set it up by doing jogless decreases as I counseled in Pretty Sleeves: Part 1, you are a good part of the way there. The keys to perfect seams are preparation and patience. You will also want either locking stitch markers or safety pins. Clover makes some great locking stitch markers, which are my tool of choice. Be sure to seam the shoulders of the garment, and to carefully block out your sleeves first.
You will be seaming each side of the cap independently, from the top down. To begin, find the exact center of the top sleeve cap. Your instructions should tell you how many stitches there are in the last bound-off row of the cap, as a check. Count carefully in from each edge of the cap top bind-off. If the number of stitches is even the center is between two stitches, if odd, then in the center of a stitch. Attach your locking stitch marker to the center of the cap top and then through the seam holding the shoulders together so that everything is lined up. Then take a second locking stitch marker and attach it to the bound off edge at the bottom of the sleeve cap, pinning that to the side seam of the sweater. Now the tricky part: find the center point between these two markers on the sleeve cap and attach that point to the center point of the armscye. Assess these centers independently: sleeve cap relative to that side of the sleeve cap, and armscye relative to that side of the armscye.
Once the two are pinned together, hold the fabric of the cap and armscye between those points (i.e. one-quarter of each the cap and armscye) together with your fingers and smooth the fabric from one end to the other from the center out to assess whether these areas can be seamed together smoothly. Do the same on the other side of the center marker, noting whether any ease on one side is equal to the ease on the other side. If it isn’t, move your center point on either the sleeve cap or armscye – whichever is making the distribution unbalanced. If you are attaching a stockinette sleeve to a stockinette body and the sleeve is standard (not pleated/puckered), you should not have to ease the fabric much to make the sleeve cap fit nicely. If you are seaming a lace body to a stockinette sleeve (as in Reverie, for example), you may have some easing to do on the lace side, which will necessarily be more stretchy.
Divide the remaining space between the center and the top and bottom points in increments of approximately 1″ / 2.5 cm by continually finding the center point between pins. Next seam the sleeve from the top center to the bottom, using a nice, even tension – not loose and not tight. I often place the sleeve over my knee when doing so (as pictured), creating a gentle curve that is similar to the curve of the fabric when worn. It helps to take out the slack and keep the sides even. While seaming, make sure that when you reach each marker you have seamed all of the fabric on both the cap and armscye sides. Because the intervals are so small, you should know if they do not match up in advance, and you can compensate for that by taking in a little more fabric on the longer side with the next seam stitch.
Repeat for the other side, seaming from top to bottom as before. Last, but definitely not least, re-block the entire garment after all of the seams are sewn. Sleeve caps will often be stiff and stand up a bit just after seaming. A good blocking will relax them, allowing them to follow the body’s natural curves.