If this is a do-it-yourself project, then you will have hopefully chosen an extremely lightweight fabric that is not stiff or starchy. A plain white cotton sheet will do, but something more lightweight and slightly more transparent will look more "floaty" and be easier to work with. This might be a lightweight rayon or cotton / synthetic blend.
Moving right along, stretch out the fabric to its full length, then proceed to attach more-or-less in the center to the round or oval head of your ghost. You can attach it with a thumb tack, pin, use a glue gun, regular white glue or double-sided tape.