Begin slowly, using a fairly mild voltage for a short duration of time to acclimate your chest muscles to the experience of electronic muscle stimulation. Aiming for five to ten minute sessions performed once daily is a good goal for you if you are a first-timer. It will take time to build up your chest, so start slow and use moderation for best results.
Gradually increase the time and the duration of the stimulation to further increase your muscular gains. Remember that the body's goal is to adapt to progressively difficult situations, so if you increase intensity too rapidly (or begin with intensity that is too high), you will have nowhere left to go to stimulate further gains. Each week, alternate between increasing the level of stimulation and slightly increasing the time of stimulation (by five minutes or so) to force your body to continually adapt to changing circumstances.
Combine your use of a muscle stimulator with traditional free weight and machine resistance training for best results. According to information from the Food and Drug Administration, although an EMS might cause some level of muscular size increase on its own, real changes to your physique are only going to be possible through the use of regular exercise as well. Therefore, consider your EMS machine as one part of your chest-buildling regime--but certainly not the be-all and end-all of it.