I am assuming you want to know how to publish a book chapter, rather than write one.
There are a few ways to go about this. The first is to approach a book publisher and inquire about writing a chapter for one of their upcoming books. This is often more difficult than it sounds, as most publishers will only consider chapter proposals from established authors with a good publication record.
If you don't have much luck going this route, another option is to self-publish your book chapter. This can be done by creating a PDF of your chapter and making it available for download on your personal website or through a third-party site such as Amazon Kindle Direct Publishing.
Finally, you could also submit your chapter to an academic journal for possible publication. This is a common option for scholars who are looking to disseminate their research to a wider audience.
Whichever route you decide to go, make sure that your chapter is well-written and thoroughly edited before you submit it for publication.