Regardless of gender, would you always use un when talking of self? For instance, Soy un cocinera mala.
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...
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