The tutorial below uses a sewing machine, but you could certainly stitch by hand if you have a nice sharp needle that will go through denim without wearing out your fingers. The project is suitable for beginners and/or untidy people.
Oh noes! These jeans have a big hole in the thigh!
Turn the jeans inside out and cut a piece of scrap denim or other heavy, non-stretch fabric. This piece is from hemming another pair of jeans. It doesn't matter if the colour doesn't match. The patch should be at least 2cm bigger than the hole in each direction. In this case, there's wear and fraying all over the inner thigh - you can see ripple marks on the fabric where it's going to rip next - so I've used a big patch to cover the entire area. Don't hem this patch - the hem will be lumpy and uncomfortable. The stitching is enough to stop fraying.
Slide a book (not a precious one!) inside the leg of the jeans. This will give you a firm surface for pinning and prevent you accidentally pinning the leg shut. Pin the patch in place on the inside of the jeans. Make sure that all holes are well covered.
Sew around the edge of the patch. Be careful not to sew the leg closed - just stop and unpick if you do! Denim can take it! My jeans are size 26 - I suspect it would be a bit trickier to wrangle small jeans on a sewing machine, but I don't know for sure. If you're using a machine, the bobbin thread will be visible on the outside, so use something not too far away from the colour of the jeans. Remove the pins.
Turn the jeans the right way out. You will see the patch through the holes. Put the book back into the jeans to give a firm surface. Pin any large holes to the patch but don't worry about pinning little holes. Don't try to close up holes if there's not enough fabric - you'll just warp the shape of your jeans. Just pin the area flat to the patch.
Sew horizontal lines of stitches across the holes, then vertical lines to give a nice tough grid. It's easiest to do this on the right side of the jeans so that you can see the holes. Make sure that you sew completely around any fraying edges, no matter how small, to stop them fraying further.
This is how it looks on the inside. It will feel a bit rough on your skin when you try it on, but it will soften up in the wash. The outside edges of the patch will also gently fray and soon you won't feel that it's there.
Presenting: my crotch! This is all that can be seen of the patch. I may have to patch the other leg at some stage, as the fabric is getting thin there, too. In the meantime, I can wear my favourite old jeans without pinching, ripping or the suffering the icy winter wind in a very sensitive place.
Crossposted to my LJ.