With these in mind, I find it very difficult to apporach these circuits shown below. What are thing I have to keep in mind solving them? And I dont know how approach these power sources.
Think of your earlier circuits as half wave rectifiers whereas your final circuit as a full wave rectified version of the voltage magnifier circuit.
Can you see that if you got rid of the two lower capacitors and the four lower diodes, you have a half wave voltage quadrupler as shown the the lowest part of your upper picture?
You get half the ripple voltage for the added complication and, for a power version of this circuit i.e. something in the order of 50 watts or above, this could be crucial for performance expectations. I had to use one like this when designing the final multiplier on an X-ray power source (50 kV at 4 mA).