Pick a smaller project, something you can finish in a few days or weeks.
For years I was thinking of creating video tutorials to teach CSS. I would sell these videos online, either as a subscription model or as a downloadable package. I spent months creating design templates that I would teach how to code. I also looked into screen recording software. I bought a microphone stand. I did some tests. But in the end, I hadn't written any content. I had only worked on the side aspects.
So one day I decided to just make the smallest project I could think of: a small ebook to teach CSS. The result was this: https://jgthms.com/css-in-44-minutes-ebook
I managed to write and launch it within a few months because the scope was way smaller, which allowed me to be more focused on the task because I could see the end of it.
As of now, I’m actually writing my second ebook, focused on advanced CSS techniques and concepts. It will have a lot more content, because I’ve learned how to launch something. The only difference is the amount of time I’ll have to spend writing. But in the end you realise that most projects scale linearly. You just need to learn how to finish a project before building bigger ones.
Same happened with Bulma: https://bulma.io
The initial idea was actually way bigger. But in the end, I launched it with the basic features I had in mind. It’s now grown a lot.