。
She tore the letter to shreds.
Literal
She [topic-は] that letter [object-を] to-shreds tore.
ずたずた is a 擬態語 (state mimetic) for something torn into many pieces, frayed, or in tatters — a visceral image of destruction. The に here adverbializes it ('to shreds'), and 裂く is a strong verb specifically for tearing along a line, splitting fibers — distinct from 破く ('rip, tear paper'). The whole describes a dramatic act of destruction, often tied to strong emotion: anger at the letter's contents, despair, or revulsion. ずたずた is also used metaphorically for something emotionally shattered: 心がずたずた ('heart in tatters').