How does a new KDP writer supposed to market a book?

Because you cannot persuade readers to buy any book in which sentences do not make sense. This shouldn't be “does.” If you don't see the problem, that is a big problem.

A) Build a following on social media, a following of people who will buy your books. Good luck with that. There is very little evidence that social media can translate to sales.

Or

I haven’t eaten junk food for weeks, I ate dirty all-day yesterday, but I can’t even workout, why am I so tired?

C) Persuade book reviewers on BookTok or Substack or someplace to recommend your book. If your book is unreadably terrible, this is probably impossible.

Are you planning to proofread your book to avoid sentences such as

B) that are filled to the brim with typos or errors.

Is it true that Jehovah's witnesses once thought the world would end in 1975?

A) that are not actually books.

How does a new KDP writer supposed to market a book?

E) Take a solid year to learn how to use Amazon or Facebook ads and be prepared to lose quite a bit of money as you figure it out.

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D) that are presented with bad covers, bad descriptions, or ludicrously bad interior formatting.

Or

D) Pay promotion services like Written Word Media to promote your book. Legitimate services will not accept your book for promotion if the presentation is crappy or the book is badly written.

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If your book is well presented, well written, and basically free of typos, then

You cannot effectively promote books

C) that are unreadably terrible in any other way.

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B) Build a newsletter mailing list of people who will buy your book because they trust you to write books they want to read. This is slow, but ideal.