First, get your victim to put on their jeans, and mark the desired hem. I'm hemming jeans for a 12yo boy - so I want them to be as long as possible while not getting walked on. I figure he's going to grow, you know?
Having marked the hem....
After I trimmed off the unwanted bits, I put a quick faux-serged hem finish on my raw edge.
After finishing the edge, turn the pants inside out and fold up the hem the desired distance.
It was at this point that I realised that it would be terminally UN cool to have a hem that far up the pant leg - so I set my hem stitch to about 1/2" from the fold. Of course I'd made a gold-colored bobbin, so I wouldn't give the whole thing away with the wrong color topstitching. (Okay. I bought the topstitching thread for my own denim project - but I'm sharing).
I then finished the jeans with a quick blind-hem to hold the top of the hem up.
And then I realised that - although I've followed the correct procedure for pants that one might wish to re-do the hem on (12yo boy, remember) I'm not sure about the quality of the fabric (see pictures one and two - the jeans developed frays during their first wash cycle. Oh well, it's never a bad thing to do things the right way - it's good practice if nothing else. And hopefully this will keep the bottom of the jeans from doing that annoying little rolling over thing that denim always does to me when I sew it myself.