I quess the question is why? The reason you are flushing the radiator will determine how far to go... If you just want to drain it to put some fresher fluid back then just open the drain petcock at the bottom and open up the top fill cap. Some places can flush the whole system so you get all of the old fluid out of the block and hoses.
Opels don't have drain petcocks...gotta pull the lower radiator hose. And to truly drain all the system, you must also remove the drain plug at the rear of the block. Of course, those are usually pretty well rusted into place.
I always add drain petcocks to my Opel radiators and also to the block. Makes it a LOT easier. I recommend it to everyone.
Regarding flushing the system, I'd also spend some time flushing the heater core. Disconnect the heater hoses and run some clean water through BOTH hoses to flush and backflush thoroughly. If the radiator is really bad, I'd consider having it professionally cleaned. Blocked passages don't cool well!
my heater core leaked like a sieve so i took it out and linked the inlet and the outlet hose together eventually I will remove all of the heater core hoses and bypass the whole thing. I wont drive it in the winter its a summer car for me, the only time i can see it to prove useful is to me is if the engine is over heating and turning the heater on to disipate heat.