Rich Snippets stars without plugin

Hello

I have an information website, but I want to place rich star snippets.

I have seen that there are many plugins that help with this function, but before I wanted to ask, can I use a schema-type code to place the stars? I guess I could do it with GP Hooks, because I want to be alone in the post.

I'm not interested in placing an average grade or grades, I want it to be completely natural, for the user.

Is it possible to do without plugin?

Thank you very much for the help.