Connect the inner part of the Leyden Jar to one pole of the Wimshurst.  The other pole of the Wimshurst is connected to the outer conductor.  Charge the jar and without discharging it disconnect from the inner conductor.  With an insulated rod remove the inner conductor from the capacitor.  You may then remove the plastic dielectric from the conductor with your hands as long as you don't touch the outer conductor.  Once the capacitor is disassembled you may handle any or all of the pieces without fear of discharging.  Reassemble the pieces and by touching the inner and outer conductors together it should produce a spark. 

NOTE: The vintage "Central Scientific" capacitor shown in picture two is made with the wrong kind of glass as a dielectric and will therefore not hold a charge.  DO NOT use this for the dissectible capacitor demonstration unless you wish to show a discrepant event type situation.