Not sure of official procedure but I recently replaced the radiator on my roundnose 04 plate.
At the top of the of the radiator there are 2x drain taps - remove the plastic cover with the MULTIPLA logo on to gain access. Theres also a drain tap at the bottom of the rad. I set the heating control to max heat and ran it for a while to ensure that the heater matrix was open. Then I opened all taps after taking the top off the expansion tank to prevent an airlock.
In hindsight I should have found out the capacity and measured exactly the right quantity of coolant I needed. However, after securing the bottom tap - I refilled the expansion tank and staredt the engine to circulate the coolant, making sure that it appeared visible through the drain tap holes. Do this slowly, switching off the engine to allow the coolant to settle until you have it to the correct level in the expansion tank.
Hope this helps.
Nick