The 12th Chapter of the Bhagavad Gita, the 'Yoga of Devotion', begins with Arjuna asking the Lord, 'Between the worship of Ishvara with form and without form, which method is superior?' Shri Krishna recommends former for most devotees as the worship of the formless is very difficult. He points out that the key quality is single-pointed devotion.
Shri Krishna then goes to list the steps to attain oneness with Ishvara. He ends the chapter with a list of thiry-nine qualities of the perfect devotee, of which attaining even one could open the door for acquiring the others.
Commentary on All Chapters of Shrimad Bhagavad Gita, available as a pack.