You don't need a template/format.  If you go to the place where they translate nepali document to English they have a template for Marriage certificate where they just write your name on the blanks.  But first you have to get a letter from your ward saying that you (Name ****) are son of Mr.**** and you married Name**** daughter of Mr*** on Marriage date.  I am not sure but I think the place is called "law book management" and it is under ministry of local development.