How to use a TENS Unit?

In order to use a TENS unit you must first prepare the area of the skin where the electrode pads will be attached. Each TENS device comes with at least two sets of electrodes, these items being the outlets through which the pain relieving properties of electrotherapy are delivered. The area to be treated may be wiped with soap and water, or with a special solution that your doctor may recommend you. Either way, you have to let it dry before applying the electrodes. If there is excess hair in that particular area, you may clip it with scissors, but do not shave it.

The next step is to connect the electrodes to the TENS unit (keep in mind that the device must be turned off during this entire preparation phase, as we may call it). The electrodes are connected to wires which are in turn plugged to the sockets on top of the unit. The TENS device has two channels; one for each set of electrodes. This means that you can use only one pair of electrodes, or two pairs on two different body parts, or all four on a particularly large area of your anatomy. 
Now is the time to place the electrodes on the skin. Electrode pads are self-adhesive, and usually come with a gel that improves conductivity and reduces the rare but potential risk of skin irritation. Use that gel. The proper placement of electrodes is essential for the success of treatment, but results vary from patient to patient, so that there is no set arrangement that works for everyone. The best thing to do is to work with your doctor and try different patterns, so that when you find the one that is effective for you, both you and your physician are aware of it and are able to recreate it afterwards. 
Depending on whether you have a digital or an analog TENS unit, you will have to manipulate buttons or knobs and dials. Either way, they should be clearly marked so that you can identify where to turn the unit on, which controls operate each channel, and how to set intensity levels and set the therapy session to a timer. Use the settings that your doctor has prescribed for you. Some units may be preset and include a safety feature that preserves those predetermined settings without risk of accidentally changing. If you own such a device, ask your doctor to preset it for you. 
The length of the session varies, but TENS units are designed to be used for extended periods. Ideally, pain relief occurs a few minutes after application has commenced, and lasts as long as the unit is being employed, and sometimes a long time afterwards. If the length of therapy is short but needs to be repeated several times, you may leave the electrodes on. Conversely, if the treatment needs to go around the clock, the electrodes should be removed at least once during the day. 
Though application may be uncomfortable, it should be tolerable. If therapy becomes painful and/or ineffective, stop treatment and consult your doctor so that different settings may be devised, or establish whether a different pain management treatment is required. Assuming that everything goes according to plan, and you have finished a session successfully, turn the unit completely off before removing the electrodes (gently lifting them and peeling them off in the direction of hair growth, as opposed to yanking them off). Replace the TENS unit and its accessories in their respective containers and cases, and store them under adequate conditions and away from the reach of children.  

Related Read: Which is the best personal TENS unit?