Nārada Muni mentions this sādhana-bhakti in Śrīmad-Bhāgavatam, Seventh Canto, First Chapter, verse 32. He says there to King Yudhiṣṭhira, "My dear King, one has to fix his mind on Kṛṣṇa by any means." That is called Kṛṣṇa consciousness. It is the duty of the ācārya, the spiritual master, to find the ways and means for his disciple to fix his mind on Kṛṣṇa. That is the beginning of sādhana-bhakti.
Śrī Caitanya Mahāprabhu has given us an authorized program for this purpose, centered around the chanting of the Hare Kṛṣṇa mantra. This chanting has so much power that it immediately attaches one to Kṛṣṇa. That is the beginning of sādhana-bhakti. Somehow or other, one has to fix his mind on Kṛṣṇa.