No documents required. Just need to fill out the form correctly and truthfully and pay the fee. Get ready for interview and prepare to answer questions such as what are their intentions to visit USA? Do you have any relatives in USA and where they live , what they do, their status etc. need to show strong relationships to Nepal and just to make them believe that they are going to return after the visit. Thats all. They already make up their half of the mind looking at the application forms. Interview is just part of the process.