Sketch a precise outline of the palette onto the drawing cardboard. Measure the circumference of your thumb and sketch the thumb hole onto the palette as well. If this is the first time you have made an art palette, it's worthwhile basing the design on a real palette to make sure the thumbhole is in the right place and the overall shape of the palette will be comfortable. Cut out the palette shape using the utility knife.
Use the cardboard stencil to draw the palette shape onto the birch plywood. Use the jigsaw to cut around the outline, don't forget the thumb hole. Bevel the edges of the palette and the thumb hole so it is comfortable to hold.
Sand the surface with a fine grade sandpaper. Next, stain the plywood with the oil-based stain. Seal with four coats of the tung oil.