First read the book completely, from cover to cover. Make sure to note down the names of important characters in the book. Also note down any new words you have learned.
Now write the review in your own words. Feel free to express yourself. Write about what you liked or did not like about the book. You may have liked the illustrations, or the writing style of the author. Please do not write the whole story. If there is a suspense in the end, do not give it away.