There is no one way to write a narrative essay on the book of Exodus. That said, there should be at least a familiarity with the book. Here is a brief recap.
There are three main sections of the book of Exodus.
First, we read of the death of Joseph and the increase of the Israelites in the first chapter. After this we read of the growing hatred of Pharaoh when it comes to the Israelites. In this section, we see the rise of Moses and his confrontation with Pharaoh. Exodus 1-14.
Second, staring from chapter 15 we read of the crossing of the Red Sea and Israel's journey in the wilderness. The climax here is chapter 19 when Moses gives the laws of God to the people.
Third, chapters 20 to the end of the book contain law and instruction on building the tabernacle.
Based on this outline, you can write an essay on various themes. Let me suggest a few. You can write about the importance of leadership by studying the life of Moses. You can also write about the importance of freedom and what that entails. Still another topic can be salvation and what that means.
Talk about Moses