A defibrillator is used to shock a heart out of arrhythmias, the most fatal of which is Ventricular Fibrillation (VF).
To use a defibrillator, put one paddle over the sternum, and the other over the right chest wall, under the axilla. Wait for the ECG reading from the machine and confirm that the heart is in VF (or some other serious arrhythmia). Get someone else to set the machine to whatever setting your local protocol indicates ...
Yell "Clear!", check that nobody is touching the patient, hold the paddles firmly against the patient's skin and press the button(s).