Today’s tutorial is one that has confused me for a very long time. It comes from Ashley at www.makeit-loveit.com.
You can find the whole post here –> http://www.makeit-loveit.com/2011/11/sewing-tip-shirringsmocking-with-elastic-thread.html
Ashley talks about getting your hands on some elastic thread
(which you may just want to get from the craft section, NOT the sewing section.) Wind the elastic on your bobbin. Use a coordinating color on top. Make lots of pretty little straight lines (or not).
There are great pictures with the tutorial at makeit-loveit, you should go read the real thing. =)