Wash the duck cloth in warm water and allow it to air dry. Duck cloth shrinks considerably, and you'll want to preshrink it before you put time and effort into waterproofing it. If you don't preshrink it, it will shrink during the waterproofing process, and the results will be unpredictable.
Stretch the duck cloth onto a wooden frame. A quilting frame is ideal for this purpose. You can use thumb tacks and tack the cloth to the quilting frame every two to three inches. Make sure the cloth is straight and squared so it doesn't pull into diagonal shapes.
Brush on an even, generous coat of shellac to the top side of the duck cloth. Work quickly because shellac dries fast. After the top of the duck cloth is dry, turn it over and shellac the back in the same manner.
Use any kind of paint (latex or oil) to paint over the shellac. You can paint a decorative design on the duck cloth or you can paint it all a solid color. If you paint it a solid color, you can speed up your work by using a paint roller. Allow the paint to dry completely.
After the paint has dried, add an additional coat of shellac on top of the paint. This coat of shellac protects the paint from flaking and chipping; it also gives the duck cloth an extra dose of waterproofing. Expect that this last coat of shellac will make your paint look a bit dull, so use a brighter color of paint than you normally would.