Who can see your posts

Turning on "Private account" removes your posts from other people's feeds and from search results.

It doesn't restrict who can open your profile or view your posts directly. There's no follower approval - anyone who follows you is added right away, and anyone with a link to your post or profile can still view it.

You need to be signed in to change this.

Go to Settings, then Privacy & safety.
Turn on "Private account". The change saves immediately - there's no separate save button.