Nikon n 80. Double exposure?

How can I do a double exposure on the same frame with the nikon n 80? Some cameras have a bottom but I can't seem how to do it with mine

Do understand double exposure? If you do you know that the camera has to shoot two shots without advancing the film.

You're correct that some cameras allow you to do this - not all but some. Have you read your owner's manual? Details are there. If the camera allows it the book will tell you how.