Ok, I shall try to explain this the best I can online.
You start with your pattern, and cut it in a line from the place where it starts to fan out into the round part of the sleeve. From there, cut four lines from the outer edge of the sleeve to the line.
The right hand picture shows the cut lines.
Then you spread the four parts of the sleeve apart, keeping the bottom corners together. There should be triangle-shapes in between each one. Retrace the pattern, and this will be your final pattern.
The red outline is the final pattern.
From here, to set the sleeve in-
Sew two lines of basting stitches on the rounded edge of the sleeve. Then pull on the top threads to scrunch it up. Scrunch it until it fits the sleeve hole, and pin it in place. Then sew it in!
Hopefully that made sense, pm me if you have any other questions!