To decorate glass Christmas ornaments, start by wiping them clean with a soft cloth and removing any dirt or dust. Then, you can apply a coat of paint or glitter to add color and sparkle to the ornament. You can also use stencils or stickers to create designs on the surface, or even add small embellishments like sequins or ribbon. Once the paint or glitter is dry, you can add a loop of string or ribbon to hang the ornament on the Christmas tree.