शनैः शनैरुपरमेद् बुद्ध्या धृतिगृहीतया। आत्मसंस्थं मनः कृत्वा न किञ्चिदपि चिन्तयेत् ॥

śhanaiḥ śhanair uparamed buddhyā dhṛiti-gṛihītayā ātma-sansthaṁ manaḥ kṛitvā na kiñchid api chintayet

Little by little let him gain tranquillity, by means of Reason controlled by steadiness; having made the mind abide in the Self, let him not think of anything.

In plain words: Krishna says to grow calm slowly, step by step, using steady understanding. Once the mind rests in the Self, think of nothing else.