Your view of the books you've read will still interest other readers. If you offer your truthful and thorough opinions, whether you've liked the book or not, then people will find new books that are right for them.
It can help to pretend you're talking to someone who asks you if they should read the book if you're stuck on what to say in a review.
Begin with a few sentences that explain what the book is about.
But without giving any plot twists or disclosing spoilers! Try to avoid writing in depth about anything that happens from about the middle of the book onwards, as a general rule. It might be helpful to note this if the book is part of a series, and if you think you'd like to read other books in the series to enjoy this one.
Discuss what you particularly liked about the novel
Concentrate on your feelings and emotions about the story and the way it was told. You may try to answer a few of the following questions:
Who was, and why, your favorite character?
Have the characters ever felt true to you?
Has the tale held you guessing?
What was your book's favorite part, and why?
Are some types of scenes especially well written - sad scenes, tense scenes, mysterious scenes, for example...?
Did the book make you weep or laugh?
Did the story grab hold of you and keep you turning the pages?
Mention all that you did not like about the book
Speak about why you don't think it has succeeded for you. For instance:
Did you wish that the ending hadn't been a cliffhanger because it was confusing for you?
Did it make it hard for you to think about the main character, and do you figure out why?
Was the story too frightening for your taste, or did it concentrate on a topic you didn't find interesting?
Round up your report
Summarize some of your ideas about the book by suggesting the kind of reader to whom you would recommend the book. For instance: younger readers, older readers, drama/mystery-stories/comedy relationship fans. Can you compare it to any books or series?
You can give the book a ranking, a mark out of five or ten, if you like!
This is a good idea. If you start, I suggest you also start a community for book reviews. I'd join it and might occasionally contribute as well.