Oh, oh! Choose ME!!! I have raised three sons and a husband, and I am TOTALLY the expert on mending jeans. I even do it for all the men in our church!
The easiest way to mend jeans is to open up the inside seam. That's usually the one that is not flat-felled. Then you take denim from other old jeans and cut a patch. If it matches well and the hole is big, I fold in the outside edges and position it on top of the jeans. Put a couple pins in it to hold it in place and stitch around the outside (folded) edges. Then stitch vertical and horizontal rows across it, like quilting a grid! Trim away any loose threads and frayed edges on the jeans. If it is a small hole, I usually put the new denim inside the jeans. Stitch heavily, like darning.
Gray thread usually works best, unless the jeans are very dark.
Then stitch up the leg again. :-)