Passive Electrodes are simple electrodes that do not have a build-in amplifier. They require very good electrical contact with the skin and therefore need careful skin preparation. The skin needs to be scrubbed until it is slightly read to get rid of dirt and old, dead skin. Then it needs to be throughly cleaned with alcohol to get rid of any oil.
You will also need to use contact gel with your passive electrodes. The electrodes should not touch the skin directly, but instead there should be a thin film of contact gel between the skin and the electrodes. For this reason most electrode disks have a small hole in them. After the electrode is fixed to the skin, the contact gel is injected by using a syringe. Also see Electrode Attachment and Active Electrodes.