In this passage, St. Faustina discusses the need for a religious to pray for a spiritual director and have God Himself choose the director. This can go for any number of things. To pray before undertaking a task for the will of God to be done, and for God to direct the task, makes a huge difference. We will not know exactly how until the next life, but it is certain.
St. Faustina's life was filled with prayer, and she composed many prayers which we have in her Diary today. Here is an example from around Christmas, 1936:
"O Light Eternal, who come to this earth, enlighten my mind and strengthen my will that I may not give up in times of great affliction. May Your light dissipate all the shadows of doubt. May Your omnipotence act through me. I trust in You, O uncreated Light! You, O Infant Jesus, are a model for me in accomplishing Your Father's will, You, who said, 'Behold, I come to do Your will.' Grant that I also may do God's will faithfully in all things. O Divine Infant, grant me this grace!" (Diary, 830)
St. Faustina was able to pray well and help us in our prayers, because she understood the things that are most important to pray for.