While patched jeans are in trend this year http://www.fabsugar.com/Trend-Alert-Patch-Jeans-6911177 and this method could work for that, but my reasons for doing this are more utilitarian. I had several pairs of jeans that my son and his Dad asked me to repair because either very worn or completely ripped out near the zipper (which is why the the pictures look different depending on which pair I was patching. No sooner did I get them patched, they wanted to wear them so I would have to start pictures with the next pair.) I used a reverse applique method instead of an over the top patch because when I asked my son which one he said a patch over the top was too obvious and people might stare at his crotch - I sure didn't want that happening!
Turn the pants wrong side out on an ironing board, if torn, scrunch the torn edges together so the edges meet. Lay a piece of the scrap fabric over the top so that it is an inch or more away from the area that needs to be patched on all sides.
Turn the patch right side up and cut pieces of HeatNBond to put around the edges only - I used scraps that were left over from other projects which is why this one looks so patchy, but it is just to keep it in place to make it easier to sew the patch down without having to mess around with breaking a needle or getting pricked by straight pins. I would not recommend putting the HeatNBond over the entire patch - I did this once and was told that it made the area too stiff and uncomfortable.
Put the HeatNBond pieces with the shiny side down, paper side up on top of the patch and use a steam iron to iron them to the patch. I use the setting a notch below the maximum on my iron, but it may vary depending on the fabric content - that might be too hot if there is a high polyester content in the fabric. Wait a few minutes to allow the strip to cool until they are easy to peel off.
After pulling off the paper, flip the patch so the right side of the patch is down on the wrong side of the jeans, position in place. If ripped, be sure to line up the edges of the rip as close as possible. If a piece goes over a seam on the leg that is not necessary, trim it off, and then iron down with a steam iron.
Turn the pants right side out and stitch around the edges of the worn area or the ripped area.
Turn the pants inside out again and if there are and loose corners or area near the stitching, trim them off.