Fold the sleeve lengthwise in half. mark that fold with a basting stitch or chalk or wash outable marker, or if the fabric will do a pressed crease.. press it.

Fold the sleeve lengthwise again, this time from the seam edge to the first fold (which should be the center line of the sleeve) mark as above..

Open up the sleeve. you should now have three vertical lines.. one down the center, and one each side about 1/2 way down the curve of the sleeve cap.

The easiest thing to do next, since you've already cut your sleeve.. is to cut the sleeve apart, down the crease lines.. you should now have four pieces.. sew a strip of fabric between each of these, reconnecting them . Adjust the shape of the sleeve & you should be good to go!

A good way to head this off at the pass is to cut a test sleeve out of scrap fabric, then "walk" the seam allowance edge of the sleeve around the sleeve allowance edge of the armscye (armhole)... that way you can make adjustments to the pattern ahead of time.. For a coat, the amount of ease (sleeve cap being bigger than the armscye) the general rule of thumb is 1 1/2-2" bigger than the armscye. Hope that helps & makes sense!

Hi, Karin-
I asume you mean that the sleeve is too small to fit into the armscye.

Have you already thought about putting in a godet at the under arm area? To make it "invisible" it should be the same fabric as your main coat. It could look like a tall triangle or an equilateral triangle, depending on how much of the sleeve does not fit, sewn into the underarm seam, short edge of triangle sews to the armscye.

If the armscye is also to small, just double the triangle so it looks like a diamond, and sew down from the armhole into the body side seam.