Disclaimer: I'm not an expert...
1. turn off boiler. You don't want hot water.
2. drain water, but don't worry about draining it all since you don't want to let lots of air into the system. The more air in the system the more it will rust.
3. Have a bucket when you disconnect the register since you will get more water.
4. Tape pipe ends to keep dirt out of the system.
1. Connect pipes.
2. Add water.
3. remove air.
Removing the air is the hard part.
1. vents and/or Valves at all the high points of the system
2. vacuum pump.
3. Enough water flow in the system to draw air bubbles to the air Airtrol vent.