Take the conductive yarn and simply tie it to the yarn that is already running into the machine. See pictures for reference. Continue to turn and the knot should be pulled into the machine and not disturb the knitting. Now knit for as long as you want the sensitive part of the sensor to be. Depending on the diameter of your knitting machine you might have to adjust this so that your sensor gets the right sensitive range you are looking for. For the example sensors (both wide and narrow) the conductive part is about 7cm long. When finished knitting the conductive part simply cut the conductive thread (only the conductive thread) and continue knitting until the remaining tail is used up.