Sometimes I think the sewing up and finishing of a garment is the hardest part. I have knit more than 10 sweaters now (10 being the number of sweaters I made before I started to consistently get the fit and style of the sweater right) and sewing in a set in sleeve is often a mental challenge. The problem with sleeves is that they are not uniform. There are bound off edges and side edges. Sleeves are curved so sometimes you are sewing a side edge to a bound off edge.
When sewing side edge to side edge I use mattress stitch. I pick up the piece of yarn that lays in between and underneath the two legs of a knit stitch and run my needle underneath it, and then do the same to the corresponding stitch on the sleeve that I’m sewing into the armhole.
2. Side edge to bound off edge
When sewing a side edge to a bound off edge, like when I sewed in the saddle shoulder tab on to the top of the One Way Tee, I use mattress stitch in the side edge. On the bound off edge I run my needle underneath the two legs of the knit stitch and I only pull tight enough so that the fabric doesn’t pucker. Some of the sewing yarn will still be visible.
3. Side edge to side edge in garter stitch
For garter stitch edges I find the purl underbumps. The underbumps are the purl bumps shaped like a frown instead of a smile. I thread my needle under one, and then find a corresponding underbump on the sleeve side and thread my needle under that stitch too, then pull tight.
I almost always have some portion of the sleeve and the armhole that need to be sewn together that aren’t the same length. After pinning the sleeve into the armhole I assess carefully. On the side that is longer, I sew two stitches to one on the other side of the garment until the length becomes even. This takes some patience. Usually I sew a few stitches together, then reassess. Then sew a few more and reassess. It is slow but I have found that any difference in length can be masked this way. The important thing is to even out any difference in length all along the seam. If you wait until you get all the way around the armhole and you have a significant difference in length in the sleeve or armhole side, and then try to sew the extra fabric in, the sleeve will be crooked on the sweater and seriously uncomfortable. When pinning the sleeve in make sure that the top of the sleeve is pinned to the top of the armhole and the bottom is pinned to the bottom of the armhole. Then add a pin or two along the sides of the armhole so that you have a good guide when you are sewing the sleeve in.