First, develop a unique alphabet or writing system. It can be inspired by existing scripts or be completely original. Then, create grammar rules. Decide on things like word order, verb conjugations, and noun declensions. For example, in the fictional language of Elvish in Tolkien's works, there are complex grammar rules. Next, build a vocabulary. You can start with basic words like 'hello', 'yes', 'no' and then expand to more complex terms. Also, think about the cultural context of the language. Where does it come from? What are the values of the people who speak it? This will help make the language more believable in the context of the book.
Well, start with a concept. What kind of world is this fictional language for? A fantasy realm full of magic? Or a science - fiction universe? Based on that, shape the language. Consider phonetics too. Decide on what sounds are common in your language. You could have a lot of hissing sounds if it's a language for a race of snake - like creatures. Regarding the book itself, you need to introduce the language gradually to the reader. Maybe start with a simple glossary at the beginning and then as the story progresses, use the language more and more in the text. And always be open to refining and adjusting the language as you write the book.