Hemming a sweater is not much different than hemming anything else, but there are a few pitfalls. You have to be careful not to let the fabric unravel when you cut it, and if the sweater is extra bulky, you could have a problem with the thick hem. If you sew the hem on a sewing machine, you also need a specialized needle for the job.
Measure the length you want the sweater while it is on the body of the person who will be wearing it. Mark the length every inch along the bottom of the sweater, using the chalk. Turn up the edge and pin it in place. Remove the sweater and turn it inside out.
Replace the sewing machine needle with a ballpoint needle specially designed for working with knits. Set the stitch to the narrowest zigzag on the machine. Ensure it is a medium stitch length.
Stitch along the marked line with a 1/4-inch seam allowance, removing pins as you go. Proceed slowly and avoid pulling the fabric through the machine. Let it go at its own pace.
Remove the sweater from the sewing machine and carefully cut away the excess sweater from the bottom, leaving 1/4 inch along the stitch line.
Sewing a hem works best on thin sweaters.You can knit a hem using a completely different method on thicker sweaters. You can hand-stitch the hem instead, using very small, tight stitches so the sweater fabric won't unravel.