Essentially, look at the rules for Cassandra's Tears or Potent Prayer and run the game like that.
Oh, that gets interesting...
- anyone with Cassandra's Tears can be compelled with the knowledge of a future event, that other people won't believe.
i) So there'd be nothing to stop you spending a Fate Point to say that another PC with Cassandra's Tears was aware of an event you are currently going through, which could be useful. "I spend a FP so that Fred knew we'd end up locked in a safe". [Fred holds up oxy acetylene torch he grabbed after seeing vision] "Fred, you realise using that in here will kill us?"
ii) Possibly the reason people don't believe the prediction is an instinctive protection mechanism to prevent them getting Lawbreaker(sixth)? In which case a PC who starts using the info gained from CT will be risking breaking the Law.
Going by our one data point in the files, the Gatekeeper' cryptic reference to Dresden, I'd suggest that you can only provide info if it is relevant to the character you are warning (thus Dresden is warned of a Doom of Damocles possibility because of his own Doom), ie you can only spend the Fate Point if it is relevant to the target's Aspects.
Should a paradox occur, the Lawbreaker Stunt strikes.
The only need for Thaumaturgy is to give yourself (or someone else) a zero cost temporary power.
Or, you could put CT and 3 levels of Lawbreaker(sixth) into an item, and the cost drops to 1 point; your crystal ball now has 3 shifts toward what you want to know. Using shapeshift to make the item out of yourself, you might be able to shed the Lawbreaker Stunt into the item (since technically you've still got it, merely trapped in the item).