6:25  

शनैः शनैरुपरमेद्‌बुद्ध्या धृतिगृहीतया ।
आत्मसंस्थं मनः कृत्वा न किंचिदपि चिन्तयेत् ॥६- २५॥

shanaiH shanairuparamed‌buddhyaa dhRRitigRRihiitayaa ।

aatmasaMsthaM manaH kRRitwaa na kiMchidapi chintayet ॥६- २५॥

Source: Automatic ITRANS Transliteration

Step by step, bringing the intellect into keeping,
Making the mind resident in the Self, dwelling on absolutely nothing [else].

Source: Milind S. Pandit

One gradually attains tranquillity of mind by keeping the mind fully absorbed in the Self by means of a well-trained (and purified) intellect, and thinking of nothing else.

Source: The Bhagavad-Gita: The Sacred Song

Gradually, step by step, with full conviction, one should become situated in trance by means of intelligence, and thus the mind should be fixed on the Self alone and should think of nothing else.

Source: Bhagavad Gita As It Is