How do I give a written answer to a petition for indebtedness?

I received a court summons from Works and Lentz in Oklahoma for monies owed to a hospital. In July I spoke with someone from works and lentz and made payment arrangements with them for 30.00 a month. I told them that I would be able to pay 15.00 out of each of my husbands checks and they said that was fine. I started payments this month and have now received the petition for indebtedness from them. How do I write the letter and who do I address it to. What information do I need to put in this letter? Thank you for all of your help.