I set them up. Add any NPC/monster block from the encounter builder. Go to that NPC's configure hero screen and add Tasha's as a source. Then go to the class tab and add any of the three sidekick classes as a class level.
If anyone can think of a simpler way to implement that, I'd love to hear it.
For future reference, any time there is a new feature that isn't obvious, I try to add an explanation in the
patch notes.