Steven Mathewson has more good advice about preaching from the Gospels. I have summarized five suggestions that he offers.
1. Read and reread the Gospels.
2. Purchase a couple of the best commentaries of the Gospel you intend to preach.
3. Do some background reading in Second Temple Judaism.
4. Select a couple of narratives and compare them in a synopsis.
5. Listen to or read good sermons on the Gospels.
Steven D. Mathewson, Preaching the Four Gospels with Confidence (Peabody, MA: Hendrickson, 2013),153-56.