Draw filing is a technique for producing a really smooth surface by using a single cut file.  By laying the file flat on the surface of the work, using both hands one at either end of the file, push and pull along the work surface.  For added surface finish, drag a piece of French chalk along the teeth of the file before using it, this will improve the surface finish and help to remove swarf from the teeth.  If you need a better finish, try using fine wet and dry sanding paper with chalk.