Firstly, understand that if you really want to get the most out of a book you will need to take time with it. You may want to read the book more than once, or at the very least reread important scenes and passages that you think are important. So be prepared to take your time with the book.
The hands down, absolutely most helpful thing to do if you're trying to read a book critically is to understand the themes and topics in the book before you start reading. There's no point in running around blindly trying to guess what passages are important in a book when a simple search can help you find exactly what you're looking for as you go through.
Remember as you're reading to take notes as you go. Don't wait until the end of the book to take note of important things that happened, because I can guarantee you won't remember them! Plus, taking notes as you go through guarantees that you're staying actively engaged in the book as you read, and increases your chances of getting more out of your read.
It's not enough simply to highlight a passage and say 'this relates to theme XXX' ... to get the most out of your reading you need to engage with the book as you go. Ask yourself why and how the passage relates to the theme in question as well, ask yourself if you agree with what the passage is implying. In short, ask questions!
Once you've finished reading the book, take a look back over the notes that you've written, and separate your notes into each theme in question. This is your chance to see whether all of the individual passages and quotes combine to paint a bigger picture, and this will be particularly important if you have to write an essay on the book in question.
Always, always, always consider the book in its context. No book is an island, there will always be aspects of the author's own life or history, or the cultural or historical context of the book to consider as well. And these factors will impact the topics and themes in question.
Don't be afraid to challenge the book! Ask yourself if you agree with what the author is saying about the topic in question, do you agree with what they're saying? Remember that the author's opinion isn't the be-all-end-all of opinions, they might be biased or flawed in their opinion, and you're allowed to disagree with them!
Above all, remember that there is no one and only way to read a book! If you don't want to read a book critically, you are by no means obliged to. These are simply here for those who are curious to try a new way of engaging with literature but don't know where to start.