Altering a cardigan sweater can be tricky and perhaps frustrating if not done correctly, because of the stretch in the knit fabric. If you are not comfortable with sewing, and especially if the cardigan sweater is one you really like, you may consider having it professionally tailored. That option will be more expensive, though, so if you have a sense of adventure and a tight budget try doing this yourself.
Wash the cardigan according to the label instructions.
Put on the cardigan and place pins where you would like the hem to be. It is easiest if you have someone else do the pinning while you wear the cardigan, to ensure the pins are even.
Take off the cardigan and measure how far up the hem you placed the pins. Write down the measurement to refer to later as you sew.
Cut down the sweater. If the difference between the current length of the sweater and your desired length is more than 1 inch, cut off the excess to avoid bulkiness in your hem. Cut the sweater with a rotary cutter (put a cutting mat underneath) or a pair of sharp scissors.
Fold the hem by covering half the distance to the desired length (where you pinned it originally) and then half again. This keeps the hem from fraying on the underside. When the hem is folded properly, pin it down. Try on the cardigan to make sure the hem is where you want it.
Load the sewing machine with all-purpose thread. Sew the hem with a topstitch by first loosening the pressure on the presser foot to decrease stretching. Topstitch with a zigzag stitch or narrow depth and medium length. Sew around the entire hem, removing the pins as you come to them.
While sewing, do not pull the fabric through the machine, it will stretch enough on its own.
If the hem turns out badly, take it to a professional tailor and have it repaired.
Alternative options include sewing the hem by hand using a catch stitch, or on a serger machine using a cover hem.
Do not sew over a pin, it will break your sewing machine needle.