I see way too many people who make grand announcements about how they're starting to build in public. I, personally, think that it's the same delusion that people have when they think that their big product hunt launch is going to blow them up.
Announcing that you're building in public means nothing.
Just go ahead and start building in public. Start writing posts about challenges that you've overcome. Post updates about what you've done to grow and further your business. Let people know what you've learned the past week while building. All of this is much more valuable content than saying "Hey, look at me, I'm building in public".
If you're REALLY building in public, people will know cause they'll see, there won't be need to announce it to anyone.