Hola Kaity

You need "un/una" when you use a sentence like you did:

Soy una cocinera mala.

But if you are just saying your profession, what you do for a living, you need to drop it:

Soy cocinera; Soy profesora; Soy ingeniera...

Saludos

Hola Carolyn 

In this specific case that Kaity mentioned, the sentence has a bit of extra information, it's adding an adjective to the profession, so the emphasis here is more on being "bad" at what you do, not so much "what you do for a living". In this case, we do need the article: 

- ¿A qué te dedicas?

- What do you do?

- Soy cocinera.  

- I'm a cook.

This is where we drop the article.

 

But if someone is saying this with more details, you do use the article:

Soy una cocinera mala. 

I'm a bad cook.

Ella es una atleta de primera clase.

She is a first class athlete.

Manuel es un carpintero excepcional.

Manuel is an exceptional carpenter.

If you read the lesson again you'll see this extra note at the end of it. 

Saludos cordiales

Inma