From my blog:
"My suggestion when putting in the sleeve is to do the sleeve cap, stopping at the small circle. Break your stitching, then clip the corner on the front and back to the reinforcing stitching, pin the sleeve from the small circle down to the side seam. By stopping the stitching this way, I got a very clean angle at that point.
Because of how insignificant the mock-turtle was on the teal sweatery fabric, I added two inches to the height of this collar. I didn't need to do that, but I do like the result. Lesson learned: If you want a collar to stand up, you've got to have some substance for it to do so.
My hems on sleeve and bottom are crisp and clean because of the 1" strip of lightweight interfacing fused to the edges before folding and double-needle stitched with wooly nylon in the bobbin."