Fill the bobbin thread with wooly nylon. I wind it on the machine but I don’t putt it through the tension when I am winding. I usually don’t have to fiddle with it since I have it on a cone thread holder but if it acts weird I just kinda hold my hand under the thread to raise it while it’s spinning. This is how I wind elastic thread for shirring too!
Use a twin needle to sew using regular (not wooly nylon) thread.
Marvel at your beautiful double row of stitching that magically has a little give making it the perfect knit hem without popping stitches or leaving you with a rippled effect.