To decorate a Christmas tree cookie, first you'll need to make a sugar cookie dough and roll it out to the desired thickness. Then, cut out a Christmas tree shape using a cookie cutter. Once the cookies are baked and cooled, you can use icing and sprinkles to decorate the cookies. For extra detail, you can add small candies, such as M&Ms or sprinkles, to create ornaments on the tree. Have fun decorating your cookies and enjoy!