How to “cleanse and purify” one’s heart and “adorn it with the spotless garment of holiness”

… love of God and spiritual attraction do cleanse and purify the human heart and dress and adorn it with the spotless garment of holiness; and once the heart is entirely attached to the Lord, and bound over to the Blessed Perfection, then will the grace of God be revealed. (Abdu'l-Baha, Selections from the Writings of Abdu'l-Baha, p. 202)