Here’s what worked for Vlad Mihalcea…
I started a blog first. This allows you to practice your writing and build an audience.
I self-published my book because publishers only wanted to give me just 10% from the profit. I used Leanpub to write and sell the book while I was still writing it and Teachable to sell it when it was done. Leanpub gives you 80% royalties. Teachable gives you around 95%.
Check his Twitter thread for the other twelve (12) things he did to make money with his book idea.