胸有成竹 (xiōng yǒu chéng zhú)

Literally: To have a full bamboo in your chest

Example: “面试前她已经胸有成竹，回答得很流利。” — She went into the interview with a clear plan in mind and answered fluently.

Origin: From Su Shi's essay on the bamboo paintings of his friend Wen Tong (Song dynasty, 11th century): to paint bamboo, Su wrote, one must first have the finished bamboo in one's chest, see it whole, and then paint it in one swift stroke. Having a complete bamboo in one's chest is having a plan fully worked out before starting.