|I use Heat'nBond iron on adhesive to help patching jeans. I also use two colors of thread to blend better for securing the patch.
The pants I show this technique on were not actually worn through, just thin. But I have successfully used this way for many patches.
I save the legs of jeans that are cut off for shorts or old jeans and try to match with the closest color jeans to make the patch. I cut out a patch larger than patch area. Then I put that piece onto the heat-n-bond, right side facing the adhesive. I cut out a matching piece and press together. Sometimes, an area will need to be cut out of the heat'n bond so it will not stick to your ironing board or anything else you don't want to stick the patch to :-)
Then I peel off the backing.
I usually put a pressing ham or roll under the patch area for easier application when I apply the patch to the inside of the area to be patched.
Once the patch is ironed on, I select TWO colors that will blend to look like the denim. Often, they will not even be close in color, such as here.
The last link also shows that I just more or less follow the grain line and stitch back and forth to secure the patch. If you select the right color threads, you can hardly even see the securing stitches.