What I would do is follow the instructions. Remove the engine fill cap, Open the heater air bleed, Add coolant to the degas bottle allowing the system to equalize until no more coolant can be added, Install the degas bottle cap, Add as much coolant as possible to the engine fill. The heater air bleed will remain open, Install the engine fill cap, Install the engine fill cap until contact is made and then tighten an additional 45 degrees (1/8 turn), NOTE: The heater air bleed remains open. Start the engine and turn the heater to the defrost (90°F) position, Allow the engine to idle for 5 minutes, add coolant to the degas bottle as needed to maintain the cold fill MAX mark, Reopen the heater air bleed to release any trapped air and close again, Maintain engine speed of 2,000 rpm for 3-5 minutes or until hot air comes from the heater, Return to idle and verify hot air is still coming from the heater, Set the heater temperature setting to 24°C (75°F) and allow the vehicle to idle for 2 minutes, Shut the engine off and allow to cool, After the engine has cooled, add coolant to the degas bottle to bring the level to the cold fill MAX mark.
You don't need to drain the coolant unless it has become contaminated.. if you only had a bad degas and replaced it I would just bleed the system following the directions listed above to THE LETTER.
Also be careful not to over tighten the caps or the bleeder.