तत्रैकाग्रं मनः कृत्वा यतचित्तेन्द्रियक्रियः। उपविश्यासने युञ्ज्याद्योगमात्मविशुद्धये ॥

tatraikāgraṁ manaḥ kṛitvā yata-chittendriya-kriyaḥ upaviśhyāsane yuñjyād yogam ātma-viśhuddhaye

There, having made the mind one-pointed, with thought and the functions of the senses subdued, steady on his seat, he should practise yoga for the purification of the self.

In plain words: Krishna says to sit steadily on that seat and focus the mind on one thing. With thoughts and senses calmed, a person should meditate to make the inner self pure.