1. Write down everything present in your solution
Eg you have K2SO4, NaOH, HCl all dissolved in water, therefore you have K+, SO4-, Na+, OH-, H+, Cl-, H2O present
2. Get your electrochemical series and circle each of those ions/molecules listed
3. Find the highest one circled on the left hand side (of course with all reactants circled), eg in O2 (g) + 2 H2O (l) + 4 e- -> 4 OH- (aq) you can't choose that because you need O2 as well as H2O
In your example this is 2 H+ (aq) + 2 e- -> H2 (g)
4. Find the lowest one circled on the right hand side
In your example this is O2 (g) + 2 H2O (l) + 4 e- -> 4 OH- (aq)
5. Check to make sure that the one on the right is higher in the series than the one on the left, otherwise it wouldn't be an electrolytic reaction.
6. Write equation out:
4OH- + 4H+ -> 2H2 + O2 + 2H2O
Voltage required is the difference between the two, ie 0.40 - 0.0 = 0.40 for this one
Cathode will have the reaction going from left -> right (gaining electrons), ie so H2 is being formed at cathode
Anode will have the reaction going from right -> left (losing electrons), so O2 and H20 are being formed at anode
Tell me if it makes sense or you need more explaining etc
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