Step 2: Harvesting the scarf fabric
If your sweater is made of a knitted or crocheted fabric, you'll want to sew a line of stitching right around the body of the garment, just below the armpits, to prevent fraying when working with the pieces later on.
Once this is done, cut through the body of sweater, one layer at a time, ABOVE the stitching, to remove the desired piece.
Finally, to make the piece workable, find one of the side seams on the sweater body and cut right along side of it, from top to bottom. You should now have one long piece of sweater with one seam in the middle of it, the remaining side seam from the old sweater.