Here is a sock that was repaired by darning.
The sock is made of wool and the heel was worn down. I looked for a wool yarn with a similar colour, but I could not find it. So, I decided to use cotton embroidery floss. Darning will not make the site of repair invisible, but it can still extend the life of the sock.
To be frank, this takes a long time to just close one little hole for me because I'm kind of slow in doing this process. So, I don't recommend hiring me to do this. There are a lot of tutorials on the Internet everywhere, so if you search with the keywords like "how to darn a hole in a sock", you'll be sure to find it. If you totally do not like thread and needle and want to hire me for extending the life of a sock, please feel free to contact me.