Incorrect spelling, explanation: itsself is incorrect because we don’t double the s when we merge it and self, despite the fact that the meaning of this reflexive pronoun is more of its plus self. Itself comes from Old English hit sylf, therefore itself is the correct form.

Correct spelling, explanation: This word is a reflexive pronoun that comes from Old English it and self (hit sylf). Despite the modern meaning being a little bit different (its own self), it is not itsself but itself. Itself is the correct form.

Definition of itself:
pronoun, the reflexive pronoun for 3rd person singular it
The dress itself is cute but it doesn’t fit you.
The company is taking full responsibility for the crew’s safety during rehearsals and the concert itself.