Trim some cones. Reserve some full-size cones. Use brush to dampen other cones with water 1 in. from wide end. Use knife to cut along dampened line. Let cone dry completely before decorating.
Decorate trees. Prepare royal icing following recipe directions. Use tip 5 and icing in cut disposable bag to attach candies to tree. Or use a spatula to thickly ice the cone and press the candies into the icing.