How to Hem Jeans & Keep the Original Hem
Start by washing and drying your jeans so that you won't have to worry about them shrinking afterwards. Next, figure out how long you want them. You can measure the inseam of a pair of jeans you love, or try them on and fold them to the length you'd like them to be. Once you've done that, decide how many inches shorter you want the jeans to be. I needed mine to be 2" shorter than what they were.
Next, fold the jeans and measure half of what the total amount of inches you'll be shortening from the edge of the original hem. I wanted mine 2" shorter, so I only folded them 1".
Iron the fold flat all the way around, measuring often to make sure you are staying even all the way around. Pin the fold in place all the way around, being careful to match up the side and inner seams. With your sewing machine, stitch as close to the original hem as possible all the way around. Backstitch at the start and finish. You'll have to go slow over the thicker seams on the side and inner part of the jeans.
Fold the extra fabric back inside the jeans and press flat, stretching the seam tight as you iron.
You can either leave the extra fabric in place, or cut it off, just make sure you have them the right length before cutting! Sometimes I leave the extra in place so that I can change the length of the jeans if I ever want to. I have a pair that I've adjusted a couple times for different shoes. Totally up to you!
Here is the finished seam. You can't even tell that they are hemmed unless you look super close.