To frost a Christmas tree, you would need to use a white, sugary glaze made of confectioners' sugar, water, and milk. The glaze should be thick enough to cover the tree but thin enough to not overwhelm the needles. Apply the glaze with a brush or spoon, making sure to cover the whole tree. If desired, you can add sprinkles or edible glitter to create a more festive look. Once the glaze has dried, the tree should have a frosty appearance, making it perfect for the holiday season.