A set in sleeve is much easier to insert when you follow these steps.

It's easy to think you can skip the hand sewing bits that look like they take extra time, but I really think they save you time in the long run, and the result is perfect easing.

The softer your fabric is, the easier this will be to do, if you're using a stiffer fabric, ideally you won't have too much easing in the sleeve head.