Mark the end you want for the sleeve - mark it all the way around. Make a second line at least one inch beyond the first. Fold or roll the excess under (inside the sleeve) and pin it. The sleeve end marked line should now be at the end of the sleeve. You can finish it by hand wit a whip stitch, or by machine with a straight stitch. Especially with the second option, try to keep the stitching line near the inside edge of the hem.
I have now showed you the, *ahem* , Hemming Way! (Groan!)