How do I activate the Z-Crystals?

All you have to do is give your Pokémon a Z-Crystal with the same Type as a move they know. When you select a Z-Crystal, your team will list if they are compatible or not. For example, a Primarina with Ice Beam, Psychic, Moonblast, and Sparkling Aria will be able to use Icium Z, Psychium Z, Fairium Z, and Waterium Z. This brings me to exclusive Z-Moves. As an example, a Primarina with its signature move Sparkling Aria would be able to use Primarium Z.