How can I get the new sona emote?

Simple answer: you dont

Full answer: you gotta find a content creator who's been affiliated with the new rito affiliate system, get INCREDIBLY lucky/be there first, get one of only like a couple hundred codes (per region), and imput it before someone else does That, or you could probably buy one off ebay, from someone who got one and got DM'd the code instead of the code being hidden in the video