Spiritual preparation is necessary to teach by the spirit

The sons of Mosiah were great gospel teachers. They prepared themselves spiritually to teach as follows: "But this is not all; they had given themselves to much prayer, and fasting; therefore they had the spirit of prophecy, and the spirit of revelation, and when they taught, they taught with power and authority of God" (Alma 17:3).

To teach by the power of The Spirit requires spiritual preparation. Prayer and fasting are part of that spiritual preparation. We pray and ask for The Holy Ghost to teach us. We fast to increase our sensitivity to the whispering of The Spirit. When we are prepared, we will teach with authority because we will know what the learners need to hear, and we will know it is true. We teach with power when we bear our testimonies.

Note that we do not prepare lessons, but rather we prepare ourselves. We fill ourselves with knowledge and truth and testimony. The Holy Ghost will teach through us, but he cannot teach from a vacuum. We must have the knowledge with which he can work. So our preparation is two fold. We enhance our knowledge, as guided by the Holy Ghost. And we tune our spirits to be sensitive to the whisperings of the Holy Ghost, so that when we enter the classroom or the teaching situation and we look into the eyes of the one we are to teach, the Holy Ghost can bring exactly the right things to our remembrance that the learner needs to hear at that moment.

D&C 11:21 reminds us,  "Seek not to declare my word, but first seek to obtain my word, and then shall your tongue be loosed; then, if you desire, you shall have my Spirit and my word, yea, the power of God unto the convincing of men." We obtain the Lord's word not only through studying the scriptures but by the communications of the Holy Ghost