Bhagavad-gita 2.39 - 2.40 Lord Krsna begins explaining working process in buddhi-yoga or karma-yoga

Working process in buddhi-yoga or karma-yoga is the practice of devotional service only for the pleasure of the senses of the Lord. 

Buddhi-yoga or bhakti-yoga is the direct communion with the Lord, who is sitting as Paramatma in everyone’s heart. Communion with the Lord does not take place without devotional service or transcendental loving service to the Lord. The stage of buddhi-yoga is attained by the special grace of the Lord only to those who are always engaged in devotional service of transcendental love. He awards such devotees the pure knowledge of devotion in love to enable them to reach Him easily in the ever-blissful kingdom of God.

In other words buddhi-yoga means to work in Krsna consciousness in the full bliss and knowledge of devotional service. One who works for the satisfaction of Lord Krsna automatically finds himself in full transcendental bliss and knowledge by the grace of the Lord. He achieves complete liberation.

In the endeavour to act for the benefit of Krsna, there is no loss or diminution. Any work begun in Krsna consciousness has a permanent effect even though not finished. Even one per cent devotional service has everlasting result such that the next beginning is from the point of two per cent. Ajamila performed specific duty in Krsna consciousness in his earlier days. In spite of leading a degraded life thereafter, he enjoyed total success at the end of life by the grace of the Lord. This is unlike material activity that requires to be 100% complete to be called successful.  

A little advancement on this path of devotional service can protect one from the most dangerous type of fear. Work in Krsna consciousness carries a person again to Krsna consciousness even after loss of body. At least one is sure to attain a human life in next birth. Such knowledge can also free one from the bondage of material activities.