RPCLI,
the document you need to complete to declare a common law relationship is called a Statutory Declaration. The clerk at your unit, or the CFRC, will have the document. It must be signed in front of a commissioned officer and it is through completion of that document that you declare that you meet the requirements for declaration of common law, i.e., that you have lived together for at least one year.
You will also need the birth certificates of your common law spouse and dependent children to have copies of them added to your file.