Combine sugar and meringue powder in a mixer bowl. With mixer on low, add the water. Beat until icing holds a ribbonlike trail on the surface of the mixture for 5 seconds when you raise the paddle.
Stir in food coloring, and use immediately; or store untinted royal icing in an airtight container (with a damp paper towel lying on top of the icing itself) up to 1 week, and color just before using.
To make the flowers: Place rose template underneath a piece of parchment paper on a flat work surface (if necessary, secure parchment to template with tape). Place tinted icing in a pastry bag fitted with a plain round 1/16-inch tip (such as Ateco No. 3), and trace flower patterns (make extra pieces, as they are fragile).
Move each sheet of parchment to a dry, flat space to set, at least overnight and up to 3 days. To remove flowers from parchment, very gently flip parchment so flowers are facing down; slowly and gently peel parchment away from flowers. Carefully lift flowers using an offset spatula, and transfer to a wide, flat container until ready to apply to cake (flowers can be made up to 5 days ahead). Do not stack.
To apply flowers to cake: Microwave 1 ounce of gum paste with 2 to 3 teaspoons water in a small bowl. Stir every 15 seconds until a gluelike paste forms (it may not be completely smooth). With a thin brush, paint the back of each flower with the gum paste, and gently apply to cake (do not worry if glue shows; it will dry clear). Hold in place for a few seconds until set.
Adorn your wedding cake with one of these thoughtful handmade touches.See the Ideas