When I first started a 2CV, I did this the way I started a modern car.

Turn the ignition key and continue starting until the engine continues to run.

However, this is not the way a 2CV should be started.

How do you do it?

With a cold engine we pull out the choke about halfway. Then we turn the key with the left hand, very short, 1 second is enough and then we release the key. 
Often the engine already runs. If it hasn't been struck yet, the choke can be pressed or pulled out slightly, that's purely on feeling.

Even with a warm engine we turn the key very briefly, a touch of about 1 second is enough!