Writing a book review follows the same pattern as any other piece of writing. Think about the pattern of writing: Beginning-Middle-End.
Write a brief description of the book. (No spoilers or plot summaries. This is not a book report).
Discuss what you liked about the book. This could be the characters, specific plot points, imagery, or any other ideas you enjoyed.
Discuss what you might not have liked. Don’t feel obligated to be negative, but if there is area for improvement mention it. If the author is self-published they may have the opportunity to fix the issues and resubmit the book.
Offer a conclusion, a summary or your final thoughts.
A few other tips:
Write the review in first person.
Be honest in your review. You are not doing the author or future readers any favors by giving a positive review when the book has issues.
Be succinct. Good reviews are between 75-300 words. (To give you a good base this document is 256 words)
Finally, books need a mixture of reviews, positive and negative. If a book has nothing but 5 star reviews they appear to be fake.
For those on the receiving end of reviews, be prepared. Reviewers will either like your book or they won’t. DO NOT respond to reviews, good or bad. If you need to read the reviews (if you’re that kind of person) then sit on them for a few days. You may find some great benefit from the information.