。
She sat with her legs crossed.
Literal
She [topic-は] legs [object-を] crossed-and sat.
足を組む ('to cross one's legs') is the standard collocation — 組む ('combine, join, weave together') applied to body parts gives the 'cross / interlock' reading. The te-form 組んで links the manner (with legs crossed) to the main verb 座った (sat down). Note that 足 covers both 'foot' and 'leg' in Japanese; context picks the right reading. Cultural sidelight: leg-crossing in formal Japanese settings is often considered impolite, especially when sitting on a chair facing someone of higher status.