Awakening the Rinnegan
To get the rinnegan you need to have the mixture of the Ashura and Indra chakras. As Uchihas are descendants of some of them and the Senju of the others, the mixture of these also allows obtaining it. The clearest example is Madara who uses the chakra of the first Hokage to obtain it.

You do not need any more requirements, but it is not a trivial process Madara itself took more than 10 years to wake up the rinnengan.

SPOILER ALERT


  Sasuke has a special rinnegan is different and more powerful, this rinnegan is called Magatama Rinnegan. He obtained it by receiving the moon crescent, I think dark, side of Hagaromo's (the Sage of Six Paths) chakra.