Google added screen casting support to a select few devices earlier this year, but the wide rollout has been very, very slow. Today there are two new devices listed on Google's screen cast support page—The NVIDIA SHIELD Tablet and the LG G Pad 8.3 GPE.
You don't need to do anything to get your tablet ready for screen casting. This is a server-side change of some sort. Both these devices should have a tile in the quick settings for screen casting, but you can also use the Chromecast app to accomplish the same thing.
I tested this on the SHIELD, and everything went as planned. It spotted the Chromecast immediately, whereas previously that button only plugged into Miracast devices.