How did Nagato gain his Rinnegan? How was it exactly that Nagato came upon his Rinnegan?

Madara states that it was his, and that he gave it to him: how did he do that? Did he transplant it into Nagato, or did he transfer it otherwise?

Note: I do not recall whether this was explained in the manga or not.
Well,


   Black Zetsu, which is the embodiment of Madara's willpower and thought (Yin), stated that he had indeed transplanted these eyes into him when he was a boy. When or how isn't stated, one can only speculate that Madara used White Zetsu or some other Zetsu for it.