Almost all of my jeans have needed to be hemmed—I'm just that kind of shape, I guess. That is, not willowy. This is how I do it so I don't lose the nicely frayed and washed-in factory hem.
1. Fold the hem to the outside so that if you were to fold just the hem back down toward the floor it would be the length you want. In this picture, the red line will be the final length of the pant leg. Press well.
2. Put a jeans needle in your machine if you have one, and an edge-stitching foot if you have one. I've recently started using the edge-stitching foot whenever I can—it just makes it easier to sew a straight line—but it's certainly not necessary here. I don't use any special kind of thread here, but you could use heavy-duty if you'd like. I used a dark blue in the needle and a dark gray in the bobbin. Using a slightly longer stitch than usual, sew right next to the turned-under edge of the original hem. Depending on the jeans and your machine, it may get tricky as you go over the side seam and inseam. Go slowly, and help it along by turning the wheel manually if necessary. If you can't get over the inseam, don't force it: cut the threads and continue by hand.
3. In this case, I did have to stop before I got to the inseam because it was too thick to fit under the presser foot, even when it was raised (these are men's jeans; I haven't had this issue with my own), so I sewed that part by hand with a doubled thread, a long needle, and a thimble.
4. Using a regular presser foot and switching the needle thread to light gray (if you want to be fussy about it), line up the left edge of the foot with the line of stitching you just made, and sew a line of zigzag stitching (scribbled in red here) all the way around the pant leg.
5. Cut off the folded edge as close to the zigzag stitching as you can, being careful not to cut the stitching and being extra careful as you cut through the thick side seam and inseam so your shears don't go where you don't want them to.
6. Sew another line of zigzag stitch that extends just over the cut edges to keep unraveling to a minimum; note that you're sewing through two layers of seam allowance here.
7. Turn the zigzagged edge up to the inside of the pant leg and the original hem to the bottom. Press the bejeezus out of it, using lots of steam.
7. By hand, tack the seam allowances up to the side seam and inseam to keep them from flopping out.
8. Press some more, and you should have a pretty good-looking hem that you'd have to look closely at to see wasn't the original. You may have to press them again after they go through the wash, you may not. You could also, I suppose, take them to a dry cleaner and ask them to hit the hems with the professional steam iron.
