|I forgot to take a picture before I mended them, but you can see with the "French" theme why like these socks.|
Then you darn the sock on the machine with a free-motion stitch. With my Singer 401, it is easy to just lift the throat plate to by-pass the feed dogs (other machines use a feed dog drop or a cover plate for darning.) Set the stitch length to 0.
You have to remove the presser foot for this to work because even with a free motion foot it will hit the edges of the frame and impede your movement. But remember you still have to "lower" the presser foot lever for it to work.
Then you start darning, by moving the work back and forth under the needle several times, then turning 90 degrees and going back and forth across it the other way.