Measure the width of the generic lampshade, available at any hardware store.
Cut out a strip of cardboard the same width as the lampshade and curved to form a quarter-circle that can be rolled into a lampshade shape.
Use the cardboard as a template to cut out similar shapes out of the decorative fabric you want to use.
Wrap the fabric around the generic lampshade and then wrap the ends of the fabric of the fabric around the edges of the lampshade, gluing them in place inside the lampshade.