How is the Rinne Sharingan awakened? How is the Rinne Sharingan awakened? I know Madara and Kaguya were the only people to do so but I don't remember it ever being explained in Naruto Shippuden.
To awaken the Rinne Sharingan I believe that you need to:

1.) possess the Rinnegan, and
2.) approach the moon/be near the moon.

Based on the Rinne Sharingan Naruto wiki page, it was written on the stone tablet handed down in the Uchiha clan that "when one who wields the power of Rinne approaches the moon, the eye which can reflect off the moon and grant the infinite dream will open". The eye here refers to the Rinne Sharingan, which Madara was able to acquire after approaching the moon.

You could read the rest on the wiki page about how Kaguya awakened it, if you would actually call that awakening. I'd like to believe that the logic behind approaching the moon to activate it was because the moon is essentially where Kaguya is, after being sealed by Hagoromo and Hamura.