Easing is slightly shrinking a seam allowance. It’s often done to sleeves. This is because the human body continues to curve out at the top of the arm, but traditionally shoulder seams sit at the shoulder tip (where the bones meet up on top of the shoulder socket). Without ease, you’d have a very tight sleeve across the upper arm.
The problem with easing is that it can be sadly frustrating to new stitchers. Here’s the least frustrating way I know that is still on the list of “right” ways to do it, and will probably not make a sewing teacher cringe. ;)
If you have a placket or other tricky bit at the bottom of the sleeve, take care of this first.
Sew a second line of stitch, just inside the first line. Yes, that’s two lines of ease stitching. This prevents your seam allowances from trying to escape.
At this point, the sleeve head is slightly over-eased. The sleeve should be smaller than the armscye. This is good, because it is much easier to adjust the two so they fit together this way than if the sleeve head is still larger than the armscye.
I think this demo was about 10 minutes (with explanation and photos) in class. One pin. No puckers, no wrinkles. (And, apparently, no picture of the sleeve. I’ll try to append this next week.
There is, of course, a faster way to get the original ease in, but it’s Cheating. ;) It’s an old trick, but a good one, and relies on deliberately throwing off your machine’s feed and thereby its tension. Ready?
If someone calls you out on this somewhat shady cheat, you didn’t learn it from me and I have no idea how these pictures got into this demo. ;)