Basically you make a little strap.

I cut my fabric 2 inches by about an inch longer than my shoulder space and then folded the piece in half like you’re making a little strap.

Sew one short side and one long side.

Right side out it and press.

And repeat for the other side.

Then when it’s time to attached the sleeve just sew in the “strap” positioning it so it just is a little longer than the finished edge of the collar if you want it to poof like mine or if you want it to be flat you can make it the same distance or a bit shorter.

Sew on a button and you’ve got a cute little should detail.