Twenty times the total amount of deuterium currently present in the world's oceans.

Basically, you'd have to completely change the chemistry of the oceans on a global scale.

Also, AFAIK there is no realistic way to synthesize deuterium artificially (please let me know if I'm wrong!) so to get started you'd need to go out and find something like an order of magnitude more deuterium than exists on planet Earth. You'd have to scour a big chunk of the solar system.

So, not something an apocalyptic cult is going to pull off in the forseable future.