Tangled Fate is an "offensive utility" spell which allows the caster to artificially change the nature of predictions upon himself or an unwilling subject. To use the spell, the caster must first ALIGN to what he wants the new prediction to be, then cast it on the subject who has the prediction he wants to change. For this purpose, the bonus derived from using Constellation Jewelry counts as a prediction.
The caster must succeed in a WvW contest to affect another person. If successful, the prediction will be scrambled. There is then a chance (based on mana for self-cast, degree of WvW win for casting on others) that the caster will get the prediction he wants. Otherwise, the result will be randomized. Especially when you cast on yourself, this is strongly
affected by your mana input: a minimum cast is simply going to randomize your prediction, while max mana provides a 100% chance (self-cast only) of success.
A subject cannot have more than one tangled prediction upon him at a time. I do not advise trying to tangle the fate of someone with more than one prediction... unless you don't like him very much.