I originally published this as part of a review of a StyleArc pattern, but I figure it will be easier to find in its own post.
How to “Flat Set” a Sleeve
First, sew your bodice front and back pieces together at the shoulders:
You can see here that the notches (I used tailors tacks) line up pretty readily. Lay your sleeve on top of your garment body with right sides together and match up all the markings. I basted the seam here, which is optional. It only adds a few seconds to the construction time and it makes everything lines up perfectly.
Then stitch or serge your final seam.
Remove the basting, press your seam and then you can sew the side seams.
Setting a sleeve this way takes much less time than the old sew-the-sides-sew-the-underarm-easestitch-and-set-in way. It works really well for knits. It also works pretty well for some stretch fabrics. It doesn’t always work for wovens (I probably wouldn’t use it on a tailored blazer, for instance), but it does for many shirt patterns. Give it a try!