I just spent the last 45 minutes trying to find a good description of the easy way to do it, and I give up. Actually - this girl does an ok job, but the video goes too slowly & skims the important bits. So here you go!
- decide/measure the length you want to shorten, and divide that amount by 2. (so, if you want to shorten by 2 inches, remember "1"). Good so far?
- fold up the jeans in a cuff until the existing hem is half the distance you want to shorten from the fold. (in the above example, measure 1 inch of doubled fabric from the fold to where the fabric touches the existing hem).
- pin in place, lining up the side hems (it will look like you've pinned the jeans in a cuffed look).
- sew as close as you can to the existing hem. It's counter-intuitive: you're essentially taking in a loop of fabric all the way around the hem.
- iron the fold of the fabric so it will lie flat.
- fold it up into the pant leg and iron again, this time along the hem, so the original hemmed edge points down, and the taken in fabric is hidden inside the pant leg.