Using a sharp knife, cut a large or small watermelon in half widthwise. Next, place the halved watermelon on its side and cut a 1-inch thick circular slice from each half.
Lay each circular slice on a cutting board and cut into six equal-sized pieces for a small watermelon, or eight pieces for a large watermelon.
To make each slice into the shape of a Christmas tree, cut off the outer edges of the watermelon rind, leaving a 1-inch “stem” for each Christmas tree. You may wish to cut the very bottom of the stem to make it flat so that it can stand up. Pat each of the watermelon trees with paper towel to soak up excess moisture.
Next, fill a piping bag with a fine tip or a plastic bag with the Greek yogurt. If using the bag, cut a very small hole at the tip of one of the corners.
Pipe the Greek yogurt onto each of the watermelon Christmas trees to form the “decorations.” You can do dots, squiggly lines, zigzags or desired decor.
Place watermelon trees flat or standing upright on a serving tray. Serve immediately or chill for later.