If you remember before bowing, say the extra takbeer if you have not said it, then recite Qunut while standing. If you remember only after entering ruku, continue the prayer. Do not return to standing to recite Qunut. Perform Sajdah al-Sahw for its forgetful omission. Hanafi ruling.

For a person praying alone, the Hanafi correction sequence is: in the final sitting recite Tashahhud; give one salam to the right; perform two prostrations with a sitting between them; sit again and recite Tashahhud, Salawat and dua; then end with salam to the right and left. Correction method.

This correction is for forgetfulness. It is not permission to deliberately leave out a necessary action. If you realise the mistake only after finishing, or have a different problem, ask a qualified teacher about your circumstances.