Answer

To set the electrolysis cells up, ready for use, try the following:
 Fill the 2 small tubes with electrolyte then insert the carbon rods which are in the rubber stopper into these tubes. The larger outer glass tube should also be in place on the stopper at this point. Apply light pressure to the bottoms of the tubes so that a seal is formed against the rubber stopper, then carefully invert the whole apparatus so that the tubes full of electrolyte are now upside down but sealed against the stopper. Now add more electrolyte to part fill (e.g. half to two thirds max.) the larger diameter outer tube of the apparatus. The applied pressure on the small tubes can now be relaxed and the electrolyte within them should stay in place and not flow out.