So from the 12v rail what should I have exactly to bring it down to 0.5 amps and then if I want later to 1.0amps

 

The resistor or lightbulb approach can limit the maximum current to those levels. In other words, the rest of the circuit can be a dead short and you will have only a targeted max current flowing through the resistor. It will drop all 12V across itself. If the rest of the circuit is not a dead short, it will draw less current. Some voltage will drop across the resistor and some across the rest.

If you want to drive a known current into a load, you need a constant current power supply. This will self-adjust voltage up to its limit, for example 12V, to drive exactly the desired current level.

If you search for a constant current power supply schematic, you'll be swamped with choices.