You use resistors on the 5V line to boost the voltage output on the 12v line (I know it sounds confusing but that's how it works)
Because you are using a resistor between
the +5v and ground wires, the less resistance in the resistor, then the closer it is to a short circuit, therefore the more load - which increases the voltage on the 12v line.
In other words to boost the voltage on the 12v line use a lower ohm resistor ie: go from 10ohms to 5 ohms etc.
Just remember that as the load increases, so does the heat generated by the resistor. Those little white block resistors may not handle the heat.
My power supply runs a 0.2 ohm 50watt heatsink encased resistor with a fan blowing on it. It will run 2 chargers plus a lathe plus run-in a motor at low voltage and the 12v line doesn't drop below 12.25v under load.
BTW there are already heaps of threads about these conversions - try using the search function