|This is a technique I learned from Peggy Sagers when she came to Boston. To ease a sleeve into a jacket, cut a 1" by 16" bias strip of tie interfacing (the loosely woven interfacing that you find inside men's ties). Place one end of the interfacing at the point or notch on the sleeve where you would begin ease stitching. Align the long edge of the interfacing with the cut edge of the sleeve fabric. Drop the presser foot on your machine to hold the strip of interfacing in place, put the machine needle down, and pull the strip of interfacing tight towards you as you begin to sew. The interfacing will narrow to a width of about 5/8". Sew slowly in a 3/8" seam, holding the interfacing tight toward you as you sew. Continue around the armsyce until you reach the point where the easing stops. When you let go of the interfacing, it will spring back to it's original width and gather the sleeve into a perfectly eased curve, which you can then set right into the armhole. The interfacing also acts as a lightweight sleeve head.
This method took me a little bit of practice, but by the second time I used the technique, I was setting in sleeves like a pro. It won't work on sleeves that have heavily gathered heads, but for tailored jackets it's a great help.
You can buy tie interfacing (trade name is Armo Rite) from tailoring supply houses, such as Oregon Tailor Supply Co. Peggy Sagers also sells it.
By the way, I have no affiliation with Peggy, I just think this is a very neat way to ease in a sleeve.