There are occasions when a stepparent may wish to apply to have parental responsibility. This may be because one of the biological parents has died or is not part of the child/children's lives.
Take the agreement to your local family court where it can be signed and witnessed.
Also take the child’s birth certificate and proof of your identity, like a passport or driving licence.
It's important to note that all the people with existing parental responsibility must agree to the additional person gaining parental responsibility. If they do not agree then you would need to ask the court to determine a decision on whether to award parental responsibility or not. If you want parental responsibility but cannot agree on arrangements with the mother, you can apply for a court order.
A court order costs £215.
Send this to your local family court.