This particular sweater project is inspired by:
image from here
-the arms of a sweater
2) Turn inside out. Pin along seam and sew in 1 inch (or whatever you measurement from above happened to be) from edge. BUT DON'T SEW A STRAIGHT LINE FROM BEGINNING TO END. Do not sew about 3/4 of an inch where you want your thumb to go. My thumb gap was about an inch from the top edge of the sleeve. (See last picture.) You may need to sew along the edges of the thumb gap if you are using a sweater that unravels easily. Mine doesn't and I like the raw look, so I left it.
3) Cut off excess sweater after sewing new seam.Craft on!