The machines are rather simple to use. First off, you need to make sure there is a full battery in the machine. Second, figure out which knob does what. The first one should change the strength of the electrical signal, while the second changes the speed making it faster or slower. Next, clean your skin with rubbing alcohol and let it dry before applying the electrodes. While it's drying, put the gel on the bottom of the electrodes, it will help the electrical signals reach the nerves.
Finally, attach the electrodes to your skin using a sticky patch or medical tape. Cover the electrode fully so it will stay in place, if you can't reach the area, ask someone for assistance. Once in place, hook the connectors to the ends and turn on the TENS unit. As you turn it on, slowly adjust the settings. Always start with your TENS machine on the lowest setting possible and raise it as needed. Your skin should feel tingly.