This is how I did it for my C3:
To get air out of the cooling system:
1) turn heater on
2) remove radiator cap
3) start car
4) run until thermostat opens
5) Watch fluid level in the radiator (ignore the reservoir). Keep the level up to the top of the radiator.
6) put the radiator cap back on
7) Fill the reservoir to the full-hot line.
If you want it warm up faster, you can pull the fuse for the electric fans. I've also put cardboard in front of the radiator to restrict air flow.
With a digital setup, you may have to start the car before you turn on the heater.
That should get all of the air out. Assuming that there is nothing blocking water flow.