You can fake a set-in sleeve by using any sleeve shape you want and inserting it into the armhole. Creating a sleeve that looks like it belonged there all along adds a soft sophistication to any garment.
For this blouse, I chose a sleeve with a slight puff (as opposed to “pouf” which conjures up disturbing leg-of-mutton images from the 1980’s...so not cool) is work-appropriate while maintaining a floaty, ladylike charm. This technique works with sleeveless t-shirts as well.
For short sleeves, buy a 1/2 yard of fabric. If you plan to insert long sleeves, you'll need at least 1 yard of fabric.