How do you become Sith? Is it only by recruitment from a previous Sith?
Dooku is regarded as Sith but what made him one? Just because Sidious said he was?
You have to learn the ways of the sith. Usually that means becoming the apprentice of one of the two sith lords but since there's only ever two that can be quite difficult so a lot of people fall into the sith by finding and studying sith artifacts.
Of course don't expect to get very far by self-studying. A good teacher is the key to your success as a Sith