Radically Candid criticism is delivered in person. Remember, Radical Candor gets measured at the listener's ear, not the talker's mouth. Since communication is mostly nonverbal, it's really hard to know if your criticism is Radically Candid -- or not -- if you can't see how it lands. The only way to know if you've been kind and clear is to see how the other person is reacting.
Watch the video to get Russ's tips for giving criticism in person:
https://www.youtube.com/watch?v=uvOrgWhVjyk
Here's one more tip to keep in mind:
https://twitter.com/candor/status/789136343301578754
Read more about giving feedback in person.