How do I sign over my parental rights to my child's aunt

Is the father on the birth certificate?
If he is you would need him to sign too.

Are you wanting her to adopt or have guardianship?

Children are not chattle that you can pass around. A parent CANNOT sign over their rights to their children Only a court can terminate parental rights. And courts are very reluctant to do so.

You MIGHT be able to have your aunt become guardian to your child depending on the circumstances. We would need to know more about them to advise.