A light metal object, for example aluminum foil with a certain shape is placed between two plates. Then potentials of different polarity are carried on the plates and the object begins to float.

With the top plate being charged to negative voltage and with the bottom plate at relative positive voltage, the electrons in the top of the conductive objectget repelled down to the bottom of the object this makes the top of it positively charged well. If the object gets too close to the plate up top electrons fly off sharp points in the bottom and it loses charge.
  Schematics for high voltage sources:

And a very important note:  Never touch the opened device when it is energised. Also wait a while after switching off to allow the capacitors time to discharge.